RE: twin motor newbie!?
That sounds like the esc is getting an irregular throttle signal. That may be your radio. RX and Tx. Are you sure you the ESC is plugged into the throttle channel? If yes do you have another RX to do a test. Maybe its your RX. I know you said your servos work but just to be sure it is not the RX do a swap and see if the same thing happens.