I'd check your fuel system to be sure you're getting enough gas to the engine during your run. I've seen fuel delivery problems cause your symptoms. If you have a fuel gauge and it drops below 5 pounds pressure during the run you need to fix it. (3/8" fuel line from the tank, electric pump, regulator, new filter for 3/8" line, float level). Will the engine RPM past 5,000 in first & second? If not you may have a valve spring problem.