I have a 92 300E that gave me a similar problem last year. Sometimes it would drop on its own or refused to go up until I shut the ignition off and a few minutes later it would work. My problem was the Convenience Control unit located under the rear seat on the drivers side.This unit controls all of your windows, sunroof and door locks. Hence the name is convenience controls.
I found it for about $250.00 and it was an OEM MB part. It is easy to put in provided you have the tech and mechanical skills. Having lived in MN for 20+ years you definitely don't need that problem this time of the year.

Good luck,

Tarheel75