Have you tried swapping the motor out with a working motor to try and localize the issue? If the problem goes away, there may be an issue with the motor, if the problem persist, there may be an issue with the speedo. Now vise versa, put the motor in your other car and see if its indeed the motors thats the issue. When were the units purchased? you may have to exercise the warranty and proceed here:
Hobby Authority

PS. And post here: https://www.rctech.net/forum/radio-e...iscussion.html next time. There are company reps that follow that thread.