VEX ?

[blackiceskier]

Also what you can try is opening up the VEX motor and make sure there is nothing in the gearing inside the motor i.e. hair or the like. We had a similar problem and it turned out to be one of the girls hair got caught in the motor and prevented it from moving on the ground but not while being held in the air. Also remember that the VEX motors are mini gear boxes check those workings often you might have snapped something.

[Al Skierkiewicz]

VEX controllers have an internal PTC type of circuit breaker. You did not mention in your post if the lights on the controller were dimming or if the controller was rebooting but I am guessing that is the problem. Most likely cause is a binding drive train. You must use the bearing plates for all shafts. You might also have misaligned gears such that there is too much friction between gears. The gears should be able to move slightly with your finger and light pressure. You may also have a bad motor(s). Does the robot run OK when you lift it up so the wheels are not touching anything?