Some of the 1980's window motors I have dealt with have a clutch set up type drive.....The idea is..when the window glass hits the top or bottom....the clutch gives to keep from frying the armature in the motor...Evidently the older motors (GM) anyhow ...had a thermal breaker inside the motor that prevented overload....Not sure.....never have had to go onto one.....But I do know the newer ones with the clutch are a big pain in the ass as they won't finish pushing the glass clear up or won't hold the glass up.....sooooo in your case I would say the clutch is the problem.....I can't give any tips on repair because I've not had any luck yet.....Find another motor or check on a reman........