Hi Derek, I cant say its true for my e34 but i had the same problem with my rover 600 which turned out to be a bent window runner membrane (metal work that does the work) this in turn changed the angle of the glass which stopped it running up the tracks properly. I would have a look at yours to see if theres any pressure on the glass as it goes up and down as this is a tell tale sign.
check the runners dont have any crap in them like moss or leaves or dirt (you would be supprised what can amss in doors).
If the glass runs fine then try some wd40 or silicon spray (have known some to use furniture polish) in the tracks where the glass runs up and down this will stop friction build up which could also be popping the window out.
if all else fails it maybe time to replace the membrane in the door most scrap yards will have these and you wont need the motor if yours is ok just the metalwork.
hope this helps
Russ