The most likely cause, in my mind, to your problem, is that your engine's carb settings are a little too lean. when you first plop it on the track and start running laps, you are immediately running fast. But as you continue to run hard, fast laps, your engine begins to overheat as the engine starts to warm up.
My suggestion is that you don't run hard for the first couple laps, to allow the engine to begin the warm up process, then tune anything you need to. Then for the next four laps, run hard and tune as needed. By the end of those six or so laps, you should get a real good idea as to how well your engine is tuned, because it has truly reached operating temperatures, before you begin to race. Otherwise, you would be just tuning in the dark.
Could be wrong, but this is the most common mistake that has symptoms simimlar to what you have said.