Sounds like a tuning issue. Mostprobably overheating just as Team17 said. Invest in a temp gun if you don't already have one. Otherwise the "spit test" will get you in the general ball park.
After the engine is warmed up and at operating temp, spit on the cooling head. If it boils of right away, it's too hot. If it sits for about 5 seconds before boiling away, you're in the ball park. If it just sits there, it's too cold (rich). When your HSN is already in a good tune, try a hole shot. If it bogs then goes, the LSN is too rich. Lean it out 1/8 of a turn. A good way to tell if the LSN is too rich or too lean is the pinch test.
Pinch the fuel line about 2 inches from the carb. If it dies after about 5 secs, you're in the general ball park. If it dies right away, it's too lean. If it takes more than 5 secs to shut off, it's too rich.