[KenWittlief]

Al has provided a great check list

and it made me think of one more possibility. You have not stated in the list of problems whether you are using any type of PID closed loop / feedback to control motor speed or robot direction

BUT if you are, and your PID loop gains are set too high, the commands going to the motors could get very wild (extreems) even if the robot is not moving around much

normally if you have the PID gain too high the robot will be twitchy, will overshoot and tend to oscillate, but you might not see those symptoms. The thing that makes this difficult to test is you cannot simply put a scope on the PWM outputs to see what the control signals are doing (if they are going wild). You would have to capture them with a printF statement or some other SW debug tool, then drive the robot smoothly and see if the motor PWM outputs are going 244, 3, 220, 12, 254, 0.... back and forth all the time.

this would be almost the same as applying full power to a stalled motor. It would draw a huge amount of current, even though the robot is hardly moving.