I had one and i had the same issue when, the timing was off or the battery was low on charge.
One other thing to look at is the timing chain. That bike had issues with the timing chain. Big time...... The my friend had one with about 25g on it and it started to cut into the head. I have heard other people talk about the same thing happening to them. Not trying to freak you out, the one i had went well over 50g before i sold it and it was still running fine.
Good luck.
G. Mountain