I went to adjust the timing on my '91, and the mark will not stay still. It is +/- about 3 degrees.

I have already replaced the timing belt, timing belt tensioner, accessory belts, distributor cap, distributor armature, water pump, fuel injectors, fuel injector o-rings and valve seals, cam shaft oil seals, all the vaccum hoses I could find, and did a complete self-diagnosis, modes 1-5 per the factory service manual.

I did all this work because the car was shaking while running (bad injectors). Now the car runs... eh, smoothly, but perhaps not as "buttery smoothly" as I might have expected after all that work.

Any other things I can look at that might be causing this?

 

Sounds like you have a vacuum leak. This will cause the fluctuation of the ignition timing at idle and a lack of your engine running "buttery smoothly".

 

stupid Q.. when you're checking timing, are you putting the car in "base timing mode" so the ECU doesn't constantly fiddle with the ignition advance? See the FSM for how to do it. I think you disconnect the TPS and IACV, then rev the car past 2000rpm for 30 seconds or something like that. it's in the manual.

If you have a shop that owns a CONSULT tool, you can also hook it up to that and it'll lock it into timing mode via the computer. works muuuuch better than trying to set the timing to a jittery mark..