It sounds like the little bearings in the gear box.... The big clue is your truck is dancing,,, that's a big sign that the bearings are shot... 2nd clue is that the truck is not going over enough to push the bit plate out far enough.... Easy fix and the bearings are readily available at any hobby shop as they are a popular sixe in the RC world... The size is 5x11x5, I've also used 5x11x4 with thin washers ( one on each bearing ).. that my current set up and has been running trouble free ...
Mans Quest for knowledge,,, means he'll always find a way !!