Start with checking for good fuel flow from the tank. Check ALL vent lines for any blockages. Drain the carb bowl into a container and look for contaminants like water. Cleaning the carb. wouldn't be a bad idea. Make sure your fuel is clean and fresh. while the carb. is off, remove the reed cage and inspect the reeds for cracks, chips, or warpage. Reassemble the bike and test ride it. If the problem still exists, check the compression (norm ave. 150 psi). Start checking the ignition system out, you may have weak spark which will breakdown at higher RPM's. There is a series of voltage and resistance test to perform on the system, refer to a good service manual for specs. and proceedures.