speed control problems

i am running an old mechianical speed control on my traxxas hawk(cars a few years old) one time i was running it the car went crazy. I unhooked the battery and found that the speed control had melted. so i put a new one on the truck and it has run fine so far but after each run i ve noticed that the positive wire hook up is really hot. could i have switched the wires or what any ideas of how i could keep it from melting would help thanks(i would buy electric if by budget premited)