350 Mercruiser straight inboard overheating

1980 Mercruiser overheating (Temp gauge pegs out and engine stalls, manifolds over 200 F) when run above 800 RPM. Century Resorter straight inboard. 350 Small V-8. Cooling water circulates, but erratically. Fresh water use only. The following actions have been taken without solving the problem:

Replaced thermostat (twice!)
Replaced water pump impellor
Checked all water lines and water intake for obstructions or restrictions
Replaced recirculation pump (After seeing YouTube video on this topic from the guy in Florida)
Removed and reground both cylinder heads.

While running thermostat housing alternates from hot to cold indicating normal open/closing, but the logs do not cool once they've gotten good and hot. The mechanic has given up! Says there's nothing more he can think of to do or check.

Any insights or suggestions?

Welcome to the forum. Did the problem begin suddenly, or build up over time? This has the Chev 350 4 barrel 255 hp. Does this have the 882 heads (last 3 digits on head casting#), and are they the thick or thin castings? The thin castings have a divot above exhaust ports, just below the valve cover surface. After 1977, the 882, at least in automotive use, became a thin, lighter weight casting, evident by these depressions. Not desirable for marine applications and prone to crack. Not knowing your boats history, things could have been changed or swapped. Air entering the cooling water could be cause of your troubles. These are really sweet boats. Is your mechanic experienced?

Thanks for the reply!
This is my 3rd Century inboard and there have been dozens in the family over the decades. Agree they're sweet! I have no reason the believe that this is NOT the original engine.
I'm afraid the boat is still at the highly experienced marine mechanic who hates inboards, so I can't check the heads right now.
I've had the boat for 4 years. I put about 10 hours on it the first year, and it ran nicely (if slightly hot) and never overheated. It had a 170 thermostat.
Year 2 saw about 10 hours of "hard" use by my kids - skiing and high RPM use. Late in year 2 it overheated while I was using it (under 3000 RPM). The temp gauge pegged out and alarm went off and I had to shut it down and swim in towing it in my teeth.
It has spent years 3 and 4 at the mechanic while he did the work I described above in three steps - returning it to me after each step. He's busy with hundreds of happy local customers and a good reputation for knowledge and honesty - although mostly with outboards. He does not hide his distaste for inboards. I can't find anyone else willing to work on inboards within 50 miles - other than the 2 big local marinas who bill at $400 per hour.

Ha sounds like a blast. So it got worse over time. I love outboards too, rebuilt my first Johnson at the age of 10 years old. Sold it for a huge profit. Probably made myself $2 an hour for my time spent. That was good money in 1966. Soon got a job at a marine dealer at $2 an hour. Went working on my own 2 years later. Did he get the heads crack tested?

Where are you located? $400 per hour? You gotta be kidding. We contract to rebuild the 8000 hp engines on the Great Lakes ore carriers. My boss charges about $450 an hour, pays us $50 each. Complete overhaul comes in at $1,500,000.00. Crew of 4 to 8, takes 7 to 8 weeks. Engines last 2 to 4 years between overhauls. Boats haul 50 to 60 thousand tons. Navy has six 700 footers in Charleston Cooper R. We changed a crankshaft on one, Lennart, (boss), charged 3 million. Freshened up all six, sea trialed the Edmont and the Domingo. They have 3 8000 hp Pielstiks.

Thanks again for your input! If an air leak in a cracked head is the only reason you can think of that would cause chronic overheating I'll be sure to check that out with the mechanic who's been working on it, but it seems unlikely he would miss something that basic. Also this would seem unlikely since both heads were just removed, resurfaced, and re-installed by a shop that specializes in that work - without any detection of flaws other than the resurfacing need. I would think they would have found any cracks while that work was being done, but I'll be sure to follow-up! And by the way, the outrageous local marina charges start with the diagnosis of a problem - $400-$800 just to tell me if they can fix it!
Has anybody else out there encountered this problem and discovered any surprising causes?

Man....I'm in the wrong business. $800 bucks for an opinion? Ha! Are you in Canada?
The "thin" #882 castings are especially suspect if they've been overheated, and yes one would ASSUME that they would have carefully checked them over. In this day and age, however, an assumption can cost some serious "bank".

are the exhaust elbows in decent shape?

They are - no restrictions or unusual corrosion.

Have had numerous different motors where the exhaust burned through to the water passage in manifolds, also in I/O drives, and in lower units of outboards. This causes the hot exhaust pressures to impede water flowing properly through the motors to cool them down. These leaks are difficult to locate, but "usually" overheating is quicker at higher rpms due to increased exhaust pressures blocking the water passage.

Just a possible thought on problem, solarman.