I think this is an easy fix. I had the same problem several years ago and it probably is a bad contact. I have a 93 850 and my window controls are in the center by the emergency brake. There are one million bolts to get that consol apart, but once you are in their, if you can spray it with some electrical parts cleaner it should fix your problem.

When someone told me this, I was sure it would not work and that I need a new motor. It fixed if for the past 3 years! I was told by a mercedes master mechanic that the swedish cars use local sources of copper that have a high tin content, which rusts over time causing alot of the electrical problems found in these cars. The japanese and germans use US copper which doesn't have the same problem. Don't know if this is true, but it fixed my issue.

Good Luck!