If it's consistently once your tank is 1/3rd empty it sounds like it could be a pressure issue. First I'd check your tank for leaks as well as your lines. Pull the tank, fuel line and pressure line. Stick a screw in the fuel line and submerge everything in water and blow on the pressure line. If you see bubbles anywhere you got a leak. If not then check your tune. Pull the air filter and set your slide so that the carb is open about 3/4-1mm. Set the HS needle flush and then fire it up n see how it idles. If you have a high idle richen the LS needle until it becomes smooth and steady (do the exact opposite if you have a low idle). Once you have that done put it on the track and run some laps only tuning the hs sothat you get the performance (and temp range) thAt you want. Then come back into the pit and see how it idles.... Now do not touch the idle stop screw but rather adjust the LS to get the idle you want (richening will make it slower, leaning will make it faster). Once you have adjusted your idle correctly with the LS you are done and your carb is set properly. After this if anything changes while running then there is something other than the engine/carb settings causing it. Hope this helps! Let me know what you come up with.

Regards,

Ron