[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?