Before you take the door apart, Open up the window switch, and clean the copper contacts with contact cleaner (any radio shack has it. Be careful not to break the plastic snaps, though. This happened to me, and it worked for a while. Eventually I needed to replace the whole switch.

Hi, I have the same problem with my '96 850. I found an article on volvospeed.com that detailed how to remove the door panel. You need a variety of torx screwdrivers and lots of time. it is not a fun procedure, but you can get access to some areas inside the door, the tracks where the window slides, etc. The article said that the track sometimes breaks and falls inside the door. I haven't been able to fix this problem. It seems as the window starts going up, the front part lags and then the back part slips out of its track. You can probably get your window up by pulling the window toward the back of the car and holding it in the rear track as it goes up. The power window mechanism appears to have two anchor points to the glass, one near the front and one near the rear at the bottom of the window. I suspect that there is some mechanical issue with the power window system. At first, I thought it was because my wife backed out of the garage and the passenger side mirror seemed to compress the front window track. I thought this was binding the window as it went up causing the window rear to go up first skipping the track.

This probably doesn't help, but I have spent hours trying to fix this, just wanted to share.