Its probably cutting out because your batteries cant supply enough amps to run that big mama jama. Go Lipo, or buy a pair of 4600 mah Duratrax 7 cell nimhs. That's probably the best nimh pack you can get for $55.
http://www3.towerhobbies.com/cgi-bin...&I=LXVBD1&P=SM

Just be sure to ditch that white Tamiya plug for a high current plug like Deans or Traxxas.