I'm not familiar with the newer Traxxas stuff, but sounds like somethings up with the speed controller. Here is a link to the Stampede section on the Traxxas website forum board:
http://traxxas.com/forums/forumdisplay.php?39-Stampede I signed up over there not long ago, but you don't have to me a member to read the boards or use the search function.
They also have a tech line where you could call & ask 'em questions, may be something simple they can help you over the phone. 1-888-TRAXXAS (1-888-872-9927) or 972-265-8000.. They are located in Plano.
At one time, Traxxas had lifetime warranty on their stuff, I suppose they still do. Didn't matter what happened to it, they'd cover it.
Sorry I can't be of more help, let us know what you find out if you call 'em!
Kenny