My engine has had two new coils and plugs, it runs beautifully up to 60% throttle, anything over that and after 10 or 15 seconds the unit starts spluttering. New tank, fuel line and fuel. Fuel filter is clean and bowl full. She runs up to full throttle in a tank and holds it no problem, but under power after 10 seconds or so she splutters......not dies. When I return to part throttle, she's fine. Any ideas?