Hi...
Being familiar (regarding Sunbird convertibles from American Sunroof) with 1987, and how those windows worked, I would say that if it is a hard stop vs a creep-creep to the last little bit that it will raise, the hard stop is likely being caused by a hard obstruction (window stop improperly installed/positioned) whereas the latter case would point me to either a weakening motor, stiff regulator, or misalignment of the door/glass against the weatherstrip along the rear edge of the glass. Of course, if the window stops in the same place with the door open, that would point to either a front weatherstrip (unlikely, in my experience) or something internal (motor or regulator).
One other thought which just occurred to me is the angle of the glass as it sits in the regulator. This can cause binding, so if you have a reference window (i.e., the other door), perhaps take some measurements partway up at the forward part of the glass vs the rear insofar as height from the top of the door.
Wish I could give you more specifics. Best would be to take the inner panel off and observe the window through an access panel with the door open. Otherwise, you'll likely be guessing, too.
Oh, and welcome to the best Pontiac-focused forums on the known Internet.
