How does the fuel look? Is it nice and clear like it was when new, or does it have a kind of cloudy look? It can't hurt to try new fuel, and if it's been sitting for a long period in a half-empty jug, it's quite possibly bad. However, bad fuel usually shows up at idle first, the high end usually isn't as sensitive to old fuel.

I agree with Dennis, I think it's a pin hole in the fuel lines allowing air in. Those pin holes can be sneaky, they are really hard to see, and they can open and close with vibration, causing all kinds of headaches.

I'm not sure if I'm reading you right, but it sounds like the engine runs fine on the ground, but dies in the air. Have you tried running a bit more of a tank on the ground at the field? Try filling the tank only half way, and running it on the ground. Do a lift test, pointing the nose up while at full throttle on the ground. See what that does.

Oh, and we have guys here using the flywheel and a standard two-bladed prop and spinner. The spinner backplate works fine with the flywheel there, as long as you buy a spinner with a flat backplate, some are recessed, while other's are flat, it's a brand thing.