Oops I hit send in error.....check the easy stuff first
I had a similar problem...your symptoms sound like mine....border line overheating from insufficient cooling can cause the problem you describe...
Check:
Mixing elbow...clean out and or replace if really clogged.they rust from the inside out and clog with carbon and rust.
Replace impeller (the rubber may be slipping on the brass hub under load) you should replace it ever year or 2 anyway....
Check/replace pump cover plate... If it's worn pump won't draw well cheap to replace ,,,remember to get a new gasket too.
Check all cooling lines to make sure they are clear. Do they leak or collapse under load....if so replace as necessary.
Check /replace thermostat ...it's cheep so I'd just replace it if there is any doubt.
Check the intake clamshell on the hull. Make sure it's clean inside and out. Small muscle shells and junk can clog it and allow enough water through to cool at low rpm but not under high.
Also check the piping from the clam shell to the thru hull valve....mine was all clogged with small shells.
Are you getting the water spitting out the rear under load. If not I would suspect borderline overheating......
IMHO.....check the easy stuff first,,,then move on to the expensive stuff.
Please let us know what you find in the end....it really helps.
