So many things...
Is the diff tight? A ball diff in a 12th car must NEVER slip. Despite a lot of rubbish to the contrary, any diff slip will give you handling problems.
Are the rear wheels at an equal distance from the centre-line of the car? If they aren't, the car will veer on acceleration.
Do the front wheels have equal 'droop' on both sides? If one has more than the other then the car will veer...
Have you checked the rear pivot balls are properly aligned and there is complete freedom of movement on the rear pod? Strip all the rear of the car off until you have just the motor plate, pivot and side links in play. Gently and slowly move the motor plate from side to side. It MUST be completely free and not notch about the middle of the movement. See the original instructions (
here) for details. This is the key to handling on a link car.
If none of this helps come back and I'll give you another list! HTH
