[Patrick Butler]

I had this same problem and it turned out the Battery mount had broke at the bottom where I couldn't see it. Everything looked fine upon inspection but once the car launched it would rock out of the mount and short out on the chassis. I assume that since you changed your batteries that this isn't you problem but it sounds like some where down the line there is a short. May have to start tracing wires to see if any are chaffing and shorting out.