Yamaha F250B overheating at higher rpms

[offshorecrazy]

I have a pair of 2008 B series F250s and one of them has a weaker stream from the check port exiting the motor. I can run them both at idle or advance the idle and run mid range for hours, run around in the bay or offshore in the 3000 - 3500 range but when I get over 5000 RPMs for several minutes, the port engine shuts down high temp/low oil pressure. A few moments later i can power up and proceed at lower speed and keep on running. I have changed the thermostats, the impeller, checked and cleaned the lower unit intakes both on the sides and the front cone. It fires right up and runs great but still the stream from the check port is not as strong as the other motor, I even pulled the zinc anodes in the head to see if something had broken off from them, they are fairly new and clean. what am I missing here? is there some way to flush the cooling system or should I just drop the lower unit again and re-check the gaskets and the plates for scarring. Any words of advice from someone smarter than me would be welcome. I had corrosion issues and exhaust leaks with an old 225, but while that would possibly explain heating the water before it gets to the engine, it would not explain the poor circulation and stream from the check port. thanks

[99yam40]

Did you replace just the impeller and not also the cup, wear plate, gaskets, o rings, and inspect housing to see if it needs replacing also?

[rodbolt17]

did you check/replace the pressure control valve?
have the ex stack checked with a bore scope.

[fairdeal]

have the ex stack checked with a bore scope.

that's a possibility, even on " 2008 B series F250" ?

ugh....