My son just opened his first xmod starter kit, a 1965 Mustang http://www.radioshack.com/product/index.jsp?productId=3307742.
We assembled it, put new batteries in all we get is steering control. It is Channel 10. It seems to be some type of radio issue because when his brother has his GTR on channel 4 running..... every once in awhile the mustang will jolt forward indicating the motor seems ok.
Any ideas of things we can try?
It sounds like there is a bad connection from the speed controller to the motor. Unfortunately, the only way to fix that is either resolder those joints or replace the wires completely. It's probably best to just return it and get a new one.