It sounds like the engine is overheating. Have you managed to run the car and finish one complete tank around the track?
It would be best to describe to us how you did the running-in of the engine.
Also, what are your current needle settings? How many turns out is the High Speed and the Low Speed needles? Is the Brass needle flushed to the back of the carb?
If in doubt about the needles, you can refer to the website below. It gives a rough guide about where and what the needles are.
Perhaps, Dino from STS can help you