Without seeing your car it's hard to tell. It is possible for the capacitor to hold a charge and cause this. Matter of fact when you solder your wires on you should get a very small arc. If you are getting more than that, I would suggest replacing the capacitor, or make sure there are no problems with your receiver. It's possible if your ESC plug is reversed or shorting somehow can cause same problem. Sounds like possible polarity problem. What kind of radio gear? Is the ESC new?