Well I changed the impeller although the old one was in good shape anyway.
Both have a brass hub.

Checked the thermostats and the pressure relief valve and they are seem
fine.

Ran the boat again today again same thing. Idle no problem. Bring up a few
revs to get 5 or 6 knots and the temp goes to the red zone. Open the
throttle a little more and still the same until the RPM is about 2000 and
the temp drops. Crank it up to 20 knots + and the temp drops to 150F and all
is well.

One thing I noticed is that the tell tale is more of a dribble now at low
speeds, and hot.



Did you ever reply as to what hull it is on?
Did you try changing the trim angle?

A cooling system can be tricky as there a a few instances where removing the
powerhead becomes necessary to check some things.
If you are not sure of what is going on it sounds like time to take it to
someone with experience.

BR