Sounds like you have a fuel restriction
check your petcock and fuel filter

a quick check to confirm the diagnoses is to run a small vacuum line to the air intake
during the power loss spray a little carb cleaner
if power improves then you are on the right track

depending on mods you could also have a carb/injector(s) issues

Hi, it sounds to me like the power reeds are sticking and will more than likely need replacing these are located between the cylinder and car its where the carb it bolted onto the cylinder these reeds should open and close to give you the power band when you need it if these stick then you get the problem your getting.