1989 Pontiac Sunbird Starter won't disengage.

(FYI - my Sunbird is the Turbo model, engine code "M") At some point after the car reached 110,000 miles, the starter began to act up. It has never failed to start the car, but it would intermittently fail to disengage. According to my Grandfather, he replaced the starter shortly after I purchased the car at which time it had around 65,000 miles on it. It was therefore a remanufactured unit, origin unknown. I never had a problem with it until now. My first step was to replace the starter with another remanufactured unit (Doh, should have just rebuilt it myself.) The one I purchased came from a store that I know to deal only with reputable parts. It costs more, but I always get original ACDelco parts made in the US. The unit is an ACDelco, but I don't recall where it was rebuilt, so I can't vouch for the quality. After installing it though, it worked perfectly, for a few weeks. Then it started doing the exact same thing as the old one. Regardless of whether the motor started, the starter would fail to disengage until I intervened. If the motor does not start, I can stop it by popping the clutch. Since the starter cannot generate enough power to move the whole car, it locks up. I don't know if it remains engaged with the flywheel or not. If the motor does start, then I can usually stop it by feathering the throttle. Occaisonally the motor must be revved up considerably, or just start moving the car to disengage, but this is normally not the case. Sometimes it disengages by itself only a second or so after the motor starts. It will continue turning regardless of what position the key is in (except I don't think I ever tried moving the key to the ACC position). Although I won't swear to it, I find it highly unlikely that the starter could have come to me with the same problem as my old one. I've played with the kill switch under the clutch pedal, and it appears to work correctly, I replaced the only relay I could find which was mounted to the firewall next to the windshield wiper motor. And I plan to check the ignition switch in the steering column next. The only other thing I can think of is shims. I have a factory shop manual for the car, and it mentions the addition or reduction of shims to correct certain problems with the starter, including a "whining" noise after the engine has started. The assumption is that if the starter is slightly out of alignment, the gear can be too tightly engaged preventing it from disengaging. According to my grandfather, as long as the gear remains in the engaged position, the solenoid will continue to supply power to it regardless of what is going on in the circuit, except of course if the power directly to it is cut. So, I guess I'm wondering, could the switch be causing this, could a relay be causing it (and if so, if it isn't the one on the firewall, where is it located), or should I just start playing with some shims for the bolts following the instructions in the shop manual. Or should I look elsewhere? And finally, although I won't rule it out, I'm going to assume that there's nothing wrong with the starter or solenoid itself, but is there any way to test this without removing it from the car (removing the starter on this car is a royal PITA, you have your choice of removing almost the entire passenger side - wheel bearing assembly, steering knuckle, driveshaft and intermediate shaft, or spend an hour+ wiggling it around until it magically fits between the intermediate shaft and oil pan. Hm, maybe I could just take the oil pan off, that's not too involved.) Anyways, I apologize for being so long but I want you to have every possible bit of information. Thanks for any and all help you can provide.