91 Excel 5spd EFI

Sounds like it could be a knock sensor or another problem -- trying to compensate for something that isn't there. I would say perhaps it is something that is checked cyclically and not continuously as you say that it comes and goes. At this point I'd go back to the basics -- compression test, and possibly the ignition module or coil. Definitely sounds like a bad misfire coming from the hunting idle, etc. How is the clutch? This can cause problems.

Thanks for the reply Littlehorn. Thought I was going to get an email from "cargurus.com" when a reply posted and I just now figured-out how to find my "question" with your reply. Clutch is great! The idle 'hunt' has been present for years. I'll read-up on the knock sensor (that's about the only thing I have'nt read-up on). Compression OK per my "car expert" (I always thought if compression was "bad" or an issue the car would run crappy all-the-time reguardless of what you may or may-not do to adjust the way it ran, at best I thought you'd have a 'always rough idle'). I'm not a car guy, but like solving problems. I'm not saying the coil is'nt the problem, but I can't understand how it would work fine when the idle is mal-adjusted (REVs 1200 to 1800 every 2 seconds repeatedly at normal operating temp.) AND does'nt work properly when the idle is at 750 (per factoty adjustment procedures). The misfire is also 99% gone when the idle is mal-adjusted and is only present when the idle is set to 750 rpm, and can be over-come (compensated-for) by changing throttle and/or gears untill misfire returns. It's possible I'm over-thinking this issue. Recently my "car expert" found the TPS had a 'bad spot' in it (near/at the beginning of it's travel/rotation close to the point were the throttle would be if/when my idle was at 2000-2500 which is where most of the 'bucking' occures when the idle is set to 750. Could my "car expert" be on to something? It's strange the heavy misfire (when I create it by setting the idle to 750) does'nt throw any codes, do you think this is some type of sign that the ECU is starting to mis-operate? Any help would be great.

ok by looking at the date of this convo it is really old but.. my excel done the same thing a couple months after i bought it and it turned out to be one of the cencers. i dont remember which one though ( it was about 2,3 years ago) but i know for sure it was not the computer

Transmisson slip.

Remove the ECM, visually check the capacitors, especially one of the larger three, the one towards the middle of the circuit board marked Rubycon 100uF 16v, this capacitor generally fails and leaks damaging the circuit board and causing miss fire, rough idle and the electric idle adjuster motor to continually cycle forward and back. It is fixed by buying a capacitor (smaller in size, but does the same job) from JayCar or similar for AU 36 cents or so. easy to unsolder remove, clean the board, resolder and then spray with electrical circuit protector to keep moisture out. These computers fail when getting a voltage shock from either jump starts, faulty batteries, alternators, starters or even some aftermarket stereo equipment.

Plug gap