The "rack" that the front door window motor gears work against is visible as you roll the window up and down. If there's a problem with this gear, you should be able to see it plain as day. If you don't see an obvious missing tooth or severely worn tooth, check the motor itself.

I went through the effort and cost of replacing both rear regulators in my '92 300SE, only to find it was the motors that were the problem.

To test, disconnect the motor from the door, leaving it plugged in. Grab the gear with a pair of pliers - not too hard as to damage the gear, but hard enough to hold tight and not let the gear slip in the pliers. Hold the body of the motor with your free hand - hold on tight. Have a friend bump the window switch up or down and see what happens. Be ready for some serious torque with this test. On my rear motors, the driver's side would work fine on the downward path, but I'd get a pop-pop-pop on the upward path. Rebuilt motor - no more problems.