Overtemp only when on plane

[lfm]

Have a 2004 Four Winns with a Volvo-Penta 5.0 GXi. It overtemps when the boat is on plane. Can run it all day plowing around at 25mph.

If I let the boat get on plane, the temperature gauge spikes quickly, so I stop the boat and push the throttle back up, but not let it get on plane and the temp drops almost as quickly as it went up.

Had the impeller replaced in the spring.

Any ideas on where to look for the problem?