50hp force high speed water not pumping

[fishheadfrog]

86 50hp force, at idle the water pumps as it should. No problems at all. Spits like a force motor should. But at some point during wot it stops pumping water. I have gave myself a head pain trying to figure this out. If any one has any ideas I would love to give em a try. Again every thing is fine and checks out good, good compression, good water flow at idle, good T-stat was removed years ago.It stops pumping at speed.