The switches are the most likely cause of the problem, but ruling that out -- and assuming you have checked/cleaned your fuses-- it could be the window motor itself is defective. You have to get into the door panel, but once in there, you can test the wiring to be sure you are getting juice. It could be you just need to clean the motor terminals, though it sounds like your motors may be going. I had to replace a motor in my '88 745 a few years back, and I don't recall it being a huge deal . . . Be sure and check that you are getting current before you go to the expense of a new window motor.