Recently, the windows on the driver's side of my '96 Outback Wagon have started occassionally violently shaking as they go up and down. Badly enough that they seem to be scratching the glass. Of course, when I went to take it in to the mechanic, they weren't doing that, and were fine for another week. Then a couple of days ago they started doing it again. At one point, the driver's window went down about 2 inches and then just stopped. (Naturally, this was at a toll both, making things very very convenient.)
When the mechanic looked at it, he said he didn't see any problems, but he did apply some lube to it.
It seems like it might be related to heat, but I'm not 100% sure. The times I had the problem, we were definitely in the 90s with very high humidity. I can't find any other pattern.