1988 LTD, 5.0
Sometimes stumbles at freeway speeds going uphill. Definitely a misfire, probably ignition. Back off the gas and it stops misfiring.

New parts (not suspected): Fuel pump, filter, tank, regulator. Cap, rotor, wires, plugs. All these have less than 8000 miles.

Old parts (suspected): TFI module and coil. Both are original. TFI has Motorcraft molded into the case. Possibly one or more injectors are acting up? But the intermittent nature makes me think this unlikely.

My gut tells me the coil isn't up to the load going uphill. Whereas a bad TFI would shut down completely.

Further info: The tach doesn't fall or twitch when misfiring, which to me further rules out the TFI module. The coil is definitely getting orders to fire.

Since they cost the same and the coil is easier to replace, I'm leaning that way, and for the little extra it costs, an MSD 8227 being American-made would give a little more quality and confidence.

FSM diagnoses the coil by using an engine analyzer (oscilloscope) which, sadly, I do not own. No codes stored in the OBD-1.

Am I on the right track here?

I would try a junk yard coil first, just because I am cheap .
But yeah I think you are on the right track.

I've had new cracked spark plugs in the past. After chasing that for a while any misfire after happens sometime after a plug change thats the first thing I will swap out a couple at a time with the old plugs. Just got to remember, you got no idea how many times they've been dropped before you got them.

pantera77 put an MSD coil on his '85 Tudor and had problems pretty directly with it (thread). He even had his issue narrowed down to TFI or MSD and it was the MSD.

Yeah, the MSD coil has some bad reviews lately. So I'll try to find a stock replacement that's halfway decent.

Update: New Standard Blue Streak coil installed. Made in Poland, has a brass terminal. Idles nice, that's all I did. Thursday I make another run thru the Valley Of Misfires.

Na zdrowie, my people thank you!

The only automotive things I know of from Poland (not counting the Poles in Detroit) are the Fiat 126 (aka Polski-Fiat, from the Iron Curtain days) and lately, Cobra replicas. Apparently there's a lot of unemployed MiG-factory employees with aluminum fab skills over there. Both have a pretty good reputation, with the caveat that the Fiat was "good for an Eastern Bloc car."

Kinda interested to see how Poland does now that they're not being invaded/exploited by their neighbors. They pretty much missed the 20th century that way.

Update: It wasn't the coil, although things generally run smoother with the new one. Still dropping a cylinder now and then on long uphills in OD. If I drop down to third, no problem.

Damn Covid has my mind all fuzzed, so I'm putting this on the back burner til I can think.

My 90 Vic is doing something similar. At idle and low rpm steady throttle it will mis fire. Normally when you stand on the gas from a dead stop and you get a miss as your accelerating under high load is when you feel an ignition miss. When you stand on jump on the gas, it is fine and hitting on all eight. I was thinking that I might have an injector giving up, or one that has a fouled tip.

I have run some Lucas Fuel injector cleaner through it, and that didn't help. Did a Sea Foam treatment down the intake, and that didn't help. So I am going to go through the plugs to rule out any issues there. I don't hear clicking or see any sparks jumping when it idles, The strange thing is that it intermittent. Sometimes it will idle good for a little bit and then you will hear it miss.
Strange.

Update again: Did a KOER test and came up with code 21, ECT out of range. That's the coolant temp sensor, in English. Replaced that, drove it around and didn't catch any misfires. BUT, the misfires happen on long hills, which are an hour away.

Since I'm still too sick to go down to Columbus and see my mom, I don't know if it's fixed or not. But initial results are pretty good.

So, drove it yesterday and it's misfiring again, not just on hills now.
Did the "watch it run in the dark" test and didn't see any sparks. But as part of that test I removed the bootie from the distributor and found six wires not as tight as they could be. That may be it, or not, but it certainly wasn't helping. Gonna blame that on bootie installation, since I know they all clicked when I put 'em on.
Next: Drive around with a naked dizzy and see.

Update: Much improved, but needs a trip thru the Valley Of Misfires to say for sure. I feel dumb if that was really it.

On a whim, I decided to replace the hose to the MAP sensor. Not realizing it's a 2-1/2 hour job because the upper intake needs to come off.

Naturally, Ferd used a special molded hose that's 1/8 on one end and 3/8 on the other end. I didn't have this hose. Probably obsoleted anyway. So I brazed up an adapter and used regular vacuum hose and power-steering hose to make it work. Taped and safety-wired in place.

Had a look at the upper-lower intake gasket. When I had the intake off before, I didn't replace it because it looked okay. This time I did replace it. There's a crush ring in the EGR passage (part of the gasket) that wants a fresh one every time, and there was a bit of oil leakage around the gasket from PCV vapors.

After a test drive, I'm calling this one done, although the Valley awaits.

Am I the only one that read that in a Russ Meyer voice doing the movie intros? =-)

I had James Earl Jones doing it.