[SoTxButler]

Something to think about...cavitation can occur when velocity of the fluid is moving to fast and reaches the point where the fluid backs up as it moves past obstructions (like sharp turns, etc.) rather than flowing smoothly. Turbulence occurs, bubbles form and so forth and so on. Overdriving the pump to this extext could be causing such cavitation and that can affect the cooling efficiency of the system.

Others with much more knowledge than me will undoubtedly shoot this down or explain it better.

Hope you can solve the problem.