On my '89 Trans am (305 TPI / 5 speed T5) that has 169k miles, I have recently felt the car 'shutter' when it is under load. It doesn't seem to matter what gear I am in or how fast I am going. On some days it will be worse than others. If I rev the engine while parked, it wont misfire or anything. But, while driving it will seem to almost 'skip'. It had sat for a few weeks and upon driving it again, this started.

Here are the things that I have tried so far:
- replaced gas with fresh 93 oct
- added 'water remover' additive to gas
- replaced fuel filter
- changed oil
- replaced spark plugs -- they did not look terrible, but had 40k miles on them.
- added injector cleaner to gas

Thoughts:
- When the issue occurs, the tachometer does not jump as I would have guessed. Does that mean it is a drive line related issue instead?
- Clutch has about 70k miles on it .. any suggestions on how to 'test' it to see if that is the issue?
- Transmission, Differential and Engine are 100% original .. with no rebuilds. Would this be an indication of transmission failure or rear differential issues? (Yes I am well aware that they ALL need to be rebuilt).

 

Could it be a warped flywheel, or hot spots on the flywheel/pressure plate. My car shutterd really bad when starting off and it was the flywheel.