okay.. about two days ago i went out to get something to eat before i had to go to work... on the 5 miles drive there my car was perfectly fine and running like it always has. after eating i hopped back in my car to go to work, as i was merging on the highway and accelerated up to speed between 50-60 mph (somewhere in there) i felt like i was driving over a rough patch of road... and on the way home from work i barely stepped on the brake and it still happened... if i put my foot down on the accelerator more it happens less. i checked all fluids on my car.. oil level normal.... transmission fluid bright red. plugged in my scanner, just the same code ive had for months now (p0420) and no pending codes... i figure it is something with the trans acting up... ive put about 12k miles on my car since owning it in a year in a half. it has almost 156k now. is there anything i can do to further diagnose the problem i am having here? i am aware that the torque converter could only act up if debris from the clutch are in there causing it to lock.. this problem has NEVER happened to me and my car has always shifted between the gears the same since ive owned it.. i thought my trans was in fairly good condition but im not sure.. any help is appreciated. im currently in the market for a 7.3 powerstroke or a 5.9 cummins diesel right now for a welding rig truck that would be my daily.. but i would like to keep my crown vic as long as possible. its always been good to be besides the few hiccups ive had. thank you

If you lightly put on the brakes (enough for the brake lights to come on, but not enough to affect load or speed) and it was still happening, it is likely engine related, either a minor misfire or something else like a vacuum leak, DPFE issue, EGR, etc, clean the MAF, etc.

Does your scanner do things like show misfire counters?

i recently cleaned the MAF when i did a full tune up. my scanner will show if i have a misfire and what cylinder but not how many times it misfires... wheres a good place to look for a vaccum leak? or would it be easier to take it to a local shop and pay the fee for the diagnostic and fix it then?

i have been scared to drive it because i dont want to be stranded anywhere. no misfire codes show up. i just scanned again and no new codes.

This is a minor issue that can be solved. How old is your fluid? A fluid swap with good quality Mercon V could take care of it.
There is also Dr. Tranny Shudder fixxx which could help. I used it in my Tbird with 200K miles and its shudder free for 25K miles.

Also, try bangsters advice with the brake pedal. That will rule out the trans.

How recent is the tuneup? Did it start after the tuneup? Pull all your coils and inspect for any moisture inside. The P0420 could have been caused by a chronic misfire.

i have already done the break pedal thing.... and the fluid is bright red still. im not sure how old the fluid is either, the tune up was done 7k miles ago with all motorcraft parts. i spent a pretty penny on them because i like to fix [censored] right and not go the cheap way so it can break again. i am reluctant to add any additives to my fluid until i get a better idea of whats wrong. also on an old transmission like mine, switching the fluid could make the problem worse... the p0420 has been there before i did the tune up. i drove for a while with a misfire and melted a cat, so its not a new code.

I drove mine for a long time before I replaced the torque converter. It didn't do any damage to the transmission, so even if the TC is the problem, it's not the major expense like a tranny rebuild. It seemed to come out of nowhere for me too. No hints of a problem until it started, and a fluid change didn't do me any good, actually made it worse. I guess my driving style was pretty easy on it. When it happened, I would let off the gas so it would only go on for like a second or so. Usually when I would get back on the gas it would be OK.

Once I changed the fluid, it was all over the place, seemed like any speed or gear. Resetting the PCM eased it a little at first and some sort of additive I got from the dealer tech did nothing. 500 miles after the change, I had to do something, as it was not getting any better.

Here is a quote from one of my posts back when I had the problem, kind of hard to remember all the details of how it was acting. Might help in diagnosing the problem.

"When its cold for the first 5-10 miles of the day, everything is fine. once warmed up, going off the dash temp. gauge, it will start some light shuddering, usually intraffic when you can't drive freely and have to maintain a set pace. If you can get away from traffic and drive a little harder, it's not so bad. Harder throttle brings on a low frequency shudder in the middle of 2nd gear. More of a pop, pop, pop, than the rapid BRRRRR of the rumble strip feeling. Normally it's gone by 45-50 mph. If cruising at 60 mph, when you floor it, it will give a thunk and a short 1 second delay before downshifting. "

I must be getting old as I don't remember some of this. I do remember it took about 3 days to replace the TC, did the J-mod at the same time. I did have a tranny jack, car was on jack stands. My assistant only gave moral support (bad back) and a loaner '02 Crown Vic for a parts chaser.

Hmmmm... So what do you think? Junk yard it when I get a new truck?

Red fluid is good, nothing is burnt so that is a good sign. However, it could be depleted of additives. If the fluid is original(which could be on a 98 because the manual says lifetime) maybe go ahead and change it. The friction modifiers break down and that is what causes shuddder. If you don't want to change the fluid add the product I suggested(do some research on it). It will replenish the friction modifiers. Lubegard is not just a junk additive.

Now, I'm saying that because it is a better option than junking the car. My 98 had original fluid, it had some minor shift issues and when I did a pan/filter drop(not a total fluid change) it shifts like a new car. Then again I took a risk and you will too by doing this...but I can show you an article written by a Ford Trans Engineer that changing the fluid and destroying the trans is an old wives tale. It will die no matter what, just a question of when.

Also, Do your homework on the product I posted.

If these measures don't work than the TC is shot.

just for future reference, about how big of a job was it to drop the tranny and swap out the converter?
i don't think i've dropped a transmission from a car before without having it attached to the motor.

If you step on the brake and it didn't quit shuddering then TC is not the problem

UPDATE: my car was sitting for 2 days and I started it up and drove for about five miles and cycled through all gears four different times, it didnt happen. This confuses me. It was just doing it but now it stops? And I will research the additive

From my experience, I used to have a 1992 GMQ and when I forced the transmission to unlock the torque converter and shift into third, this cause the converter to "chatter." Is this what you did when you merged on the highway?

Sounding more like fluid issues. Friction modifiers can change with heat.

A great upgrade would be the 11.25" torque converter. You will have to replace the flex plate as well. The 98 P71 was the last year for P71 to receive the 12" torque converter.

Get grab a 99 P71 ARA3 PCM and MAF sensor and be on your way.