I had something stupid happen to me one time and it took awhile before I figured it out. I had run my fuel line through the cockpit along the bottom frame rail. It ran right next to the throttle pedal. Well, apparantly it moved over time and everytime I raced, the motor would fall flat after 3-4 laps. I was actually pinching the fuel line with the pedal. The more I pushed on it to go the worse it ran. Get back in the pits and check it out and was always fine then. Ran new hose and re-routed it just a bit.

I hope you find the problem. I know what a headache it can be.