Barry,

Slowly leaning the high-speed needle and having the engine suddenly jump in RPM and run very lean is a very strong indication that you are getting fuel foaming.

Fuel foaming is when the engine's vibrations agitate air into the fuel that's in the tank. You won't likely see bubbles in the fuel line because they'd be very small. The engine vibrates heavily because you are likely getting valve float, which means that the intake and exhaust valves can't move quite as fast as the engine's demanding, and they neither open or close fully.

You'll have to either add more foam rubber around the tank or remove some. Too little foam provides no vibration insulation, and too much foam does the same thing because the foam's too densely packed.

To confirm the situation, you can remove the engine from the model and put it on a solid test stand that won't allow much vibration, and where the fuel system is completely isolated from the engine's vibrations. The engine should perform normally.