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

Derek, I had the same problem. The rear edge of the window popped out when about halfway up, leaving an annoying wind noise in your ear.  Having had the door apart on several occasions to no avail, I eventually sorted it using two methods. I greased the rear edge black felt runner with some clear Vaseline (pop some in with a small screwdriver). It didn't leave any marks on the glass, although Russ' suggestion of silicon spray might be a better option. I also inserted a u-shaped wedge of electricians tape at the front (the short vertical section of runner near the door mirror) - lots of layers of about 1" of tape, then bent into a U.

Try the lubrication method first.

Derek

I've had the same problem too.

In my case it was the window runner itself.

At the bottom of the runner, if you pull the rubber out you will find a torks bolt that holds the runner to a bracket spot welded to the door.

On mine the weld had broken, so I drilled the weld out, drilled and tapped the bracket, and secured it from the outside of the door using a stainless 3mm countersunk cap head.This is covered by the rubber on the door.

Thanks for the replies,

sounds like one for tackling over the weekend.