i bought a new shaft for mine when it started to **** up cost me $30 bux on ebay. worked great for almost 2 years now, but it started doing the same thing again since it got really cold. i figure it has something to do with the temp and weather.
ive been looking for a good lubricant to coat it with, but it has to conduct good so no luck yet.

btw that thumping is the teeth on the shaft skipping inside the motor, usually just means the antenna has to much of a bend and the cold doesnt help either. i noticed driving fast has negative effects on the front license plate and the antenna. my front plate went from being brand new to having a 30* down bend in it toward the under bumper scoop. so you can imagine the bend the antenna can get over time.

~boom