The impeller and housing are cheap... With all the issues you've had they are likely damaged at this point, 4bus is right! Start there by simply replacing them. Also a lot of guys have mentioned running a hanger or something through to clear any blockage a general flush won't get, do that and replace the thru transom hose. Whole deal won't be $100 and you'll know for sure if it's your pump or not when you are done.

Are you absolutely sure you have the right impeller shaft diameter on your impeller? It sounds like the rpm causes the shaft to start spinning inside the impeller..

The impeller was replaced in June. That's why we were looking elsewhere. Trying the new pump in a few minutes.

While you have it off do yourself a favor and backflush the oil cooler and lines that go to the pump from the thermostat housing before you put the lines back on just to make doubly sure there are no old impellar pieces in there.

Did all of that. The lines are all clear.

I installed the other pump, fired it up on the hose and waited. Pulled the thermostat inlet hose and had plenty of water. Increased rpm, still plenty of water. More rpm, plenty of water. I quit at 3k. Lots of water. Looks like a bad pump. I'm going to take it apart to see what's what. For now, it would appear to be fixed.

Thanks for all the suggestions everyone!

change seals on old pump housing. check shaft for grooving. previous impeller may have knicked seal.