First: make sure your top deck is aligned properly. The plastic pieces that hold the bellcrank shaft are NOT symetrical.
Second: Make sure that your diff is not angled. It could be if you put it in and didn't pay close attention to the belt tightening adjustments on both sides of the diffs (the black plastic peices that notch into scallops in the bulkheads)
Third: Make sure that there is no binding in the drive shafts, like the bearings are going out on one side, or if your using blades on the outdrives-that one of them isn't broken.
Fourth: Check for tweak in the chassis. Loosen the screws in the top plate, and the screws that attach the bulkheads to the bottom chassis, and retighten. Bottom screws first, then press down on a flat surface and tighten the top deck.
If you go through the car complete, and can't find the problem, it could be some type of a glitch with your electronics.