engine stumbling/vibration

He could be having a rich misfire at that range, which could be many of problems (which would explain the gas/oil concoction he drained from the oil pan). I am leaning toward a computer programming problem. the computer could have a program error allowing the injector pulse width to be to high for that range causing a rich misfire condition (hence washing the cylinder walls). It could be something totally different too, that is just what came to the top of my head for some reason. I'm guessing since it's a GTA that it is an older firebird no? I'm not familiar with a GTA mustang from that era (I could very much be wrong however as I am newer to the mustang world), but the guys at www.thirdgen.org could be of much help to you, specifically a guy named vader as he knows chevy/gm engines inside and out. I would give a little more help, but it could be a little harder to diagnos without a scope to put the car on.

As far as the compression deal, it really wouldn't matter what the rpm range was, it would affect all ranges. Incidentally, I just tested a car with 70% leakage (not confirmed but all evidence points to rings, short of tearing the car apart) and it was running perfectly with no misfire, not that it couldn't happen because anything is really possible.