Most likely the plastic "rollers" inside the window motor gear assembly. 3 out of 4 windows on my 94 Vert were doing that when I bought it. I just used some properly sized metal nutserts that I had in my shop. Added a little duct tape around the nutserts to take up slack and to also put a little "give" between the pieces. At least for a little while, anyway.
I cleaned up all the plastic powder that was crunched up in there, greased it up and put it back together. Works like new. Didn't cost me a thing but time. Besides. Any use of duct tape in the repair of a car increases the likelihood that it won't break again.
The teeth on the regulator arm are metal. Even though I have seen a photo of them worn/broken on here, I will still say it is exceptionally rare since the plastic bits are the weakest links and I believe Ford put them in there as a wear item. And possibly to act as a sort of "fuse". What I did was the equivalent of wrapping a blown fuse in aluminum foil... I shall accept my consequences.
The pieces in question would be those little cylinder bits in 74409.