200 opti loses speed and dies under a load...

[Lizzy K II]

I'm completely stumped on what could be causing my motor to die out after about a minute of running under a load. I've checked all fuel pumps, filters, and for leaks. fuel pressure stays up as well as air pressure. DDT does not find any problems. what Would could cause my motor to run normal at an idle and to die after running at a higher speed? As soon as it dies it will crank up again. I spent all night last night diagnosing a air leak in the system but I fixed that before heading down to the dock and running the motor under a load. Any ideas? It seems i've checked all the obvious culprits...