I would be more than happy to help you out, just let me know. I know when my 250 did this, it was an ESC issue. Try this. take the pinion off your motor, and plug the ESC into the elevator spot on the Rx. You will have to put the stick all the way up or all the way down to keep it from spinning, but you can test if it's your Rx not sending the proper signal. I don't think it's gears or the belt since the motor would put so much power (I noticed full stick when doing this) that it would strip out the main gear, and would not jerk.
If you are using a DSM2 or DSMX Rx, then I won't need the Tx to test it, I can just use my DX8. But if you are using a different Tx, then I cna either use one of my Rx's, or you can send your Tx. Either way, if you decide to send it to me, just let me know. I hope you get it figured out soon though cause shipping a heli is expensive and doing it twice is even more so lol. Just let me know.