I just encountered this problem in my 87 300E. My tech did a 2 minute diagnosis - he said start with the simplest parts first. He tried the switch, said he heard a slight normal contact noise which lessened the chance of a bad switch in his mind (for the first go round). He held the switch in the open position while he had me firmly close the door with the offending window. The window started working, and has worked ever since! (this happened once before, but apparently had solved itself through door closing). He said this indicated several possibilities:
1) the wiring to/in the door had a problem
2) the lift motor had a flat spot
3) the rubber oxidation on the top of the window (thin black horizontal line) was 'gluing' the window up - I had this problem on my acura - cleaning the top of the glass solved the problem).
4) their was a mechanism bind starting to occur.
Since it is working, he said live with it until it starts to be a problem, then start with the cheapest and simplest fix (test the switch and wiring thoroughly before buying a new motor or playing with the mechanism).
BTW, great points on the AC and guides. I am approaching 70k miles and use zero oil between changes at 3 k intervals, so I guess I'm lucky. I would suggest checking for frequency of brake line and coolant flushes - they hadn't been done on mine and it cost me a water pump. It has been a great car for me (The Sportline suspension transformed it completely!!!) I plan on saving a little each month for when it needs the AC, guides, timing chain, etc. and keeping it until it rusts out from under me!
87 300E, 65k miles