Originally Posted by Demothen

Unfortunately I don't know which wire is which, however once you get that figured out, it sounds like you might be drawing too much current due to load on the motor. There's a thermal/current draw overload switch inside the motor housing that might be getting too hot and breaking continuity. On my '85 w123, I have refurbished 3/4 window regulators by removing them, cleaning all the nasty grease off the gears and linkage, and re-greasing them. I also had quite a bit of flex in my regulators, so I ended up flattening them out by clamping them to a 2x12 and flexing the aluminum until it was back to a reasonably flat shape. You might also grease the window rails. I used a combination of silicone spray for the slides and thicker silicone grease for the mechanism.
A simple test to see if the overload switch might be causing the problem is to thump the motor while applying voltage. This rattles the switch and seems to have a fairly decent chance of fixing the problem short-term. You can even just slam the door while holding the switch, if you don't want to remove the door panels.