New to electrics, so hope this is just a nube issue!

setup:

eflite 370 outrunner 1080
Castle C. 10 - ESC
Kokam 3sp1 pack (780mah)

First flight things went fine... nosed over on landing, then the problem starter.

Now when I try to apply throttle the motor jerks back and forth like it can't make up it's mind which direction to turn? Happens at ANY throttle position... If I leave the throttle on for a couple of seconds the ESC just shuts down the power to the motor.

any ideas???