OhSnap,
Had the same problem and did all that you did and then some.
Problem lasted two years and $1200 later.
Untill I found the cause. It was the flaps inside the exhaust just past the manifolds. BOTH flaps , that prevent sea water from entering the engine were stainless steel, but BOTH flaps were damaged.
I had my son take it out while I would shot the piping and mainfolds with infra red temp sensor and would see the port get hot while the starboard side would stay cool. The next day just the opposite would happen.
I replaced these two flapsl($35.00 each) that the two years of nusiance overheating problem that you exactly described, was resolved.
I just wish someone would have checked these out sooner.
Took sometime, but that what it was.
They tell me they last about 10 years...my boat is a 95 Maxum.
Check these flaps and see if they are in working order.
I'm guessing they aren't...My mechanic also said, (after I told him that I found the problem) , that many people are clueless aboout those flaps , they break off and slide down .
Only when they block the flow are they noticed because of the overheating.
Good Luck
