The idle could be one thing as mentioned in the previous post but something else to keep in mind is the needle positioning. A very common mistake is people will run the hsn too rich and to compensate, they lean the lsn to clean up the bottom end but as the engine builds heat, the engine naturally leans itself out and if you are running too lean on the lsn, the engine builds even more heat in the infield and cools down in the straight away but over time, the cooling effect will not be effective and the engine will tend to flame out when the slide closes as the heat buildup in the carb from the lean condition evaporates some of the fuel entering the case and the hotter the engine and carb gets, the more fuel and the quicker it evaporates until there isn't enough getting to the chamber to sustain continuous operation.

If the engine is doing it right out of the gate, try raising the idle a little and/or going to a hotter plug but if it runs good initially and it is somehting that happens several minutes into a race, retune the engine so the lsn and hsn receive proper fuel delivery at all times.