Mine does the same thing...The felt that is in the rubber channel that the window rides up and down in is worn away...I replaced it and all is well....The channel was applying to much friction to the glass and was slowing in down...
Dumb question, but did you check voltage and grounds. On my anny it ran slow and it was the ground. On the 89sc it was the switch not allowing full voltage. Corrected groung on anny and replaced switch on 89. Fixed both.