Hi,
1: Check the spark plug gaps, if the gap is too small it will run ok at low revs but stutter and missfire at high revs.
2: Check that the float or needle valve in the carburettor is not sticking. If it doesnt open properly, fuel starvation happens at high revs under load.
3: Does it have a choke for cold starting. It it sticking on, this causes rough running in the higher rev range.
4: It there an in-line fuel filter that needs cleaning out, it maybe starved of fuel.
5: Does the fuel filter inside the tank need cleaning, again fuel starvation.
6: Check out the air filter, might be choking her.
Good luck mate!
Steve