I have the same problem. I test everything. And I mean everything. Same thing happened to my buddies islandia and it was his hi pressure fuel pump inside the vapor separator tank.
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Basically what he explained to me is that the pump gets weak with age and pumps enough fuel for idle, but not enough when the engine is under a load.
The only other info I could find is that the fuel injectors are dirty. I will try the pump first since it is the cheaper option. If that doesn't work then I will change the injectors.
If you come up with some different solution before then please let me know. Thanx, Eric!
I will be changing mine by next weekend and will let you know what I come up with.