I had trouble a while back with electrically-noisy Tamiya/Mabuchi motors resetting a low-voltage Pololu motor controller. Soldering 0.1uF caps from each motor terminal to the metal case (after lightly sanding two spots on the case) solved that problem. Some people also recommend bridging the two terminals with 0.1uF caps, though I didn't bother.
See here: Pololu - 9. Dealing with Motor Noise