From the start, sorry about writing a novel. Trying to be as precise as possible.

I'm gonna start from the beginning. Everything is just as usual, I'm on the highway making my 30 mile drive to college. Just when my exit comes up, i start slowing down from 70mph... and feel something strange in the car's feel. Right away I shut off the stereo and vents to minimize confusion. It feels like its shuddering. I get to my first red light, idle is steady at 700rpm in D and brake applied... sounds normal and then again shudders on and off at random times. Now i'm freaking out, it feels like a "cammed" car, then smooth as ever. On and off. Pull in to the first service station I see and pop the hood. I'm staring at the engine and I can see it run smooth... then it jumps o so slightly, but noticeably, on the car's lateral axis. Now I might add it's snowing and I have a bad hand so I was pretty frustrated at this point that I can't check very much. So I check my fluids, coolant is low (surprised me) but engine was running, i'll have to check that tomorrow morning. Usually that has always been full to the MAX line and it was about 3-4 inches from the full mark. Engine oil was a little low, added some. Couldnt check tranny with my bad hand. Quick visual inspection, everything looks OK...

This went on all day, only seems to be around at low RPMs. Idle to around 1800 RPM. But the car doesn't feel like it lost power... I floored it on some open stretches and it feels fine.
No CEL(and it's not burnt). Gauges are normal. RPM doesn't vary with the "shudders".
Fuel filter is not even a year old. I always run 91octane. Spark plugs changed last year. COPs maybe?? but then why no CEL... Engine mounts?? vacuum leak? but then RPMs are steady.

Now. I just got home and it's a nice and quiet street. So I throw it in Park, no brake. Smooth... AH there it's still doing it. so I go to the back of the car and listen to the exhaust. Usual flowmaster sound and then when the vibration/shudder occurs it sounds like pop-corn in the exhaust. So when the "problem" happens, inside the car you feel a vibration/shudder and a sound like corn popping in the exhaust.

All this and no CEL.

I am stumpped and freaking out.
One thing that crossed my mind but I don't know how possible this is. Maybe the plastic coolant crossover finally gave way just a bit and coolant is getting sucked into the engine? that would explain low coolant and the odd running. I really have no idea.

Thanks in advance for any help/ideas.
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1998 Silver Frost P73 / 2003 White P71(not running yet)

-91 octane tune by BOC, MM airbox + MAF, PI Intake swap, BBK 70mm TB, ported intake elbow, 22C plugs, Walbro 190, 180 T-stat, EGR delete, Jmod, Flowmaster 40s, P71 springs and shocks, slotted brakes, MMC drive-shaft, 3.55 posi w/ carbon clutches, brand new Motorcraft COPs and headlight relay upgrade.
http://www.youtube.com/watch?v=ZDD0f3KhIL0
http://www.youtube.com/watch?v=iimDSfeaupY