Did this problem happen progressively or all of a sudden? It's evidently starving for fuel, or sucking air. Sounds like you have eliminated fuel blockages or air leaks in the fuel supply system.
Some questions that come to mind:
- by fuel supply system does that include the pressure line? And checking the pressure tap on the pipe that it is clear?
- if it happened between one season and the next, corrosion taking out a bearing? (Do you even have flying seasons? - dumb Canuck) As thailazer suggested perhaps one has gone bad for some reason, could happen gradually or all of a sudden.
- are you eating plugs?
- if you're not, how old is the plug?
- leaky header/cylinder seal?
- leaky pipe coupler?
Hmm.. what else..