I'm trying to troubleshoot this annoying shudder that happens to my 91 305 formula TPI...basically when your holding a constant speed the car will begin to making a "thunk" noise and jerk back and forth. It's still drivable, but its annoying. You can feel it through the shift stick. Also it sometimes stumbles slightly when accelerating, but this is not a big deal. In addition, the car will really decelerate when you let off the gas in lower gears, the front end dives right down.

So far I have replaced the cap, rotor, plugs, wires, O2 sensor, coolent temp. sensor, PCV valve, and have new bosch 22# injectors. Fuel pressure is well within range, vacuum is 18 inches and timing is dead on.

My next move was a new EGR valve but I'm thinking it might be a driveline issue? When this happens the revs don't change, not too a degree that you'd notice on the tach anyhow. U-joints appear fine and when I changed the diff fluid, the gears looked intact and in good shape.

Any ideas?

take ur harmonic balncer bolt off and put locktite on it then torque thats what my old tbi one did the ses light would come on too and if u look at it while it does the bucking the harmonic balncer is actually moving out throwing off the timing

Did you ever figure out this issue? I am having the same problems. Changed the plugs / fuel filter so far .. going to check the wires / cap / rotor tonight.