"What RPM's are you runnin
"What RPM's are you running at Wide Open Throttle (WOT)? If you are going past 6,000, the rev limiter will kick in and the engine rpms will start to fluctuate.
If you punch it to WOT and it runs smoothly for little while, you have a fuel restriction. The engine will run good until it uses up the gas in the carb bowls, then will start to starve for fuel. If you punch it to WOT and it starts to miss right away, then some of the small holes in the main jets are clogged up. The carbs have to come off, be disassembled, and cleaned out.
Either that or you may have some"