3.0L overheating

Cannot solve overheating problem on my 3.0L 140R engine with Mercruiser Sterndrive. Rebuilt the sterndrive pump last year and it puts out plenty of water flow to the engine. Put on the earmuffs in the driveway and water temp will stabilize at 140 deg. F. Plenty of coolant flow exiting the sterndrive. Took it out in the river and water temp will stabilize only below 2000 RPM, which is before the boat is even on plane. Once RPM exceeds 2000, the water temp continues to rise. Put the boat up on plane and run @ 2500, the temp continues to climb and within minutes is heading north of 230. Removed thermostat just to make sure it's not the issue and there was no change. Checked all hoses, engine water pump, and cannot find any issue. The engine water pump is original and appears to be in good condition. This boat is a 17' bowrider and used in fresh water only. Any ideas? Thank you. Fred

2 things to check, does the hose from the circ pump to the T Stat get hard with the rpm?
Yes, clogged,restricted riser.
High speed overheats always start with the impeller, recheck your work. Including the base of the pump

Thanks BT Doctur. I will try the hose test next time out on the water. As far as the impeller, are you referring to the sterndrive impeller?

yes, impeller in the drive