It would be much easier to diagnose what's wrong if you explain exactly what's it doing or not doing. Does it creep at all when the petal is pushed 1/4 of the way? Forward and reverse? Does it shut itself off (pmt)? If so, what are you doing exactly when it shuts down? How does it react as you slowly press the petal down? Can you press the petal down all the way and have it click into high speed and run? Does it run differently with the wheels spinning off the ground rather than driving it? It shouldn't be difficult to figure out what's wrong but you have to be crystal clear in explaining how it is reacting while trying to drive it.