offtraxx,
If you are using the standard 20 tooth pinion that is provided in the kit, over a period of time it will wear and then damage the spur gear in the car. When the pinion wears the teeth will be curved on one side and resemble the shape of a shark fin. The standard pinion is made from soft aluminium and is normally one of the first parts I replace during assembly.
As 1101 as said, the hardened 20 tooth pinion is the one to get. I would also check to see if you have any wear/play on the spur and intermediate gears or that the chassis halves mounting points have any play. I would aslo check all of the 1150 and 850 bearings.
Just my thoughts,
Calvin.