I had the exact same problem, and lived with it for a year or so, and then finally got around to replacing the regulator. In my case the teeth on the regulator were ground almost completely down. I didn't see any sign of cracking of the regulato mechanism, but it does flex some when you keep holding the switch button down at the top of travel. In fact, the flexing was starting to tear the door sheet metal at one of the regulator mounting bolt locations. To reinforce it I added some large washers.

The good news is that the teeth on the motor pinion gear are of a more robust material, and stay good. The regulator is a lot cheaper than the motor. It is a bit of a pain to do the replacement, but when you are done you'll wonder (like I did) why I lived with the "thump-thump-thump" for so long. The only other thing I vaguely remember is that I ended up having to cut the electrical connection (wires) to the motor. I can't recall if I absolutely had to do this, or if it just made it easier to get things out of the door. But it wasn't too bad reconnecting the cut wires afterward. I soldered them, rather than just using wire caps.