Quote Originally Posted by tom4bren View Post
Cavitation. Been through it with the impeller on my SeaDoo. It runs fine at idle but more & more efficiency is lost as the speed increases. Basically the pump could be causing an air pocket to form in the pump restricting flow at higher rpm whilst at idle everything is working fine.

The reason that we usually don't see erosion of the pump blades is that the seals usually fail long before that occurs.

I'm not sayin that your pump is bad ... it's just one of the possibilities to consider.
I agree with Tom that this is probably a water pump issue. I've heard of similar problems when air is the system- caused by cavitating or not. I would start by burping the coolant system of air. There's a couple easy ways to do this and it's a good place to start. If the problem is fixed but comes back, then it might be time for a new water pump.