Mike,


It looks to be a fuel system problem.
Please inspect the clunk line inside the tank.

The engine can become lean when there is a pinhole in that fuel line. as the fuel level goes down, part of that line becomes exposed to air inside the tank, effectively leaning the mixture.

Also, is your fuel tank completely isolated from vibrations?
If any part of it is in direct contact with the air-frame, the fuel will foam, with the same results...

Make sure there is a 1/4" of latex foam, between the tank and all adjacent parts of the tank compartment, including the brass/aluminium fuel tubes that exit the tank.