If it's well lubricated, and has been sucessfully initilized with the problem still occurring... It will need a new window motor. I've seen this several times on 1ers..
'11 135i DCT - Stock - Traded in.
'97 M3/4/5 - Boosted.
'98 Z3 2.8 Roadster - AKG poly everything, GC coilovers. 220k and climbing.
'13 Smart Fortwo - DD/beater.