Sounds like the "safety-releases" or whatever they are called inside the driven gear in the window motor gearbox have broken.

It has been over 20 years since I fooled with them, but it was a pretty common failure, the poor mans cure was to replace the safety gizmo inside the gear with something unbreakable, I used short lengths of appropriate sized copper tubing, about 1/2" long, I think maybe 3/8" tube? Maybe bigger ...

Pull the motor from the regulator, remove the cover, then the gear ... you will see what is needed.

I have no idea if the factory (or aftermarket) parts are available, but those motors were pretty much the same from at least the early 60s thru I think the 80's.