read the throttle addendum on Losi's website, follow it EXACTLY and your troubles should be over.
I have gone through several sets of o-rings, and just leave the screw a little loose. There are several theories about what the real problem is (heat, vibration, EPAs). I believe it is the End Point Adjustments, and it seems more and more people are thinking the same thing. Your servo wants to move further than the carb and the brakes will allow it, there for you have to set your radio, not to allow the servo to travel so far; EPA. When you read the addendum, it will explain how to leave adjust both.

http://www.teamlosi.com/ProdInfo/Fil...e_Addendum.pdf