i know it seems kinda silly to point out but just in case. you are aligning the set screww to the flat part of the motor shaft so it stays in place right? also check that your pinion has not cracked right where the threads are, if it is it will loosen also. then a new pinion would help you out. or that you haven't stripped the threads trying to tighten it up to much. again a new pinion will help.