This, I think, is going to be one of those faults where you're going to have to trace things out and look for faults. Could be the switch...a continuity check will tell you about that. Could be a wire problem...that's going to take some tracing and looking. Could be the motor going out...apply 12VDC to it directly and see what it does. It could even be the window armature or tracks sticking. Just open it up and take a look.