So it runs fine with nimh but not with the lipo? That is very odd. You say you checked the voltage of the lipo and it is above 7V? I am not all that familure with the sidewinder so not sure if there may be some other setting in there that my be messing with stuff.

Sensored motors have a separate sensor wire that plugs into the back of the motor. I am assmuing your setup is sensorless though.