At wits end......

describe the stumble, is it stalling/quitting - fuel, or breaking up/stuttering - ignition. check the reluctor to pickup gap, if it is too small it will break up at high rpm. also if your stock fuel inlet hose is 5/16" , the outlet should be no larger than 5/16". stock 80,000 mile springs should go to 5500 rpm all day with that cam, make sure there are no broken springs