99% of carb problems are ignition related. (that's a joke, but usually true.) The problem may be weak spark caused by a carbon burn path to ground, or short circuit. Have you gone over your dist, rotor, coil, and cables? Maybe the spark becomes so weak after warming up that you have to keep the rpm's up to stay running. Does it backfire when you rev it up? That's a sign of weak spark because unburned fuel collects in the exhaust and then ignites. Have you tried a C-Spark device to check the strength of the coil voltage?