Hi,
could be ur engine is running too hot.
In July, my sceadu 50 Heli had that problem. i posted a thread like urs, on a heli site. I got an aftermarket shroud that blows air over the 50 head. Problem solved. In fact i could lean the motor out more.
I'm not saying u need a shroud. I think you need to investigate how to cool ur motor a bit to stop it overheating.
KYOSHO 1