2004 Crownline 210 w/ 5.0 Volvo Penta GXI overheating alarm

We've been dealing with an overheat alarm with our boat for a while now. The boat runs, temp gets to 200, and the alarm goes off, putting boat into 'limp home' mode. We're stumped as to what's causing it. We have used a heat sensing gun to verify the temperature on the engine - of note, the ONLY spot where it's hot is by the top of the exhaust (near thermostat), everywhere else is cool. It does seem to 'cool off' pretty quickly after it overheats, but will then warm up to 200 again fairly quickly as well.

Here's what's been 'fixed' (replaced) so far:

impeller
water pump
thermostat

heat sensors were also checked (not replaced), as they are working
boat mechanic also checked for any leaks within the exhaust system and didn't find any

Based on isolated top of exhaust location of the 'overheat', we next did a pressure test on the 8 valves to see if a blown head gasket was a possibility - all 8 valves were within Volvo's "okay" range, but 1 valve was close to being outside of it. Based on this, we did a full head gasket replacement. After that was done and we tested it, I could tell we had been running on 7+ cylinders and the engine had more pop, but we still got the over heating alarm within 10 minutes of operation.

Any ideas for what to look at next would be greatly appreciated!

Re: "top of the exhaust"... not a good enough explanation ( at least for me!).... WHERE exactly. Based on this limited information I would speculate that this exhaust manifold is not getting full water flow. As I recall (Ricardo, catch me if I'm wrong), even FWC ( has heat exchanger) Volvo V8s have raw water cooled exhaust manifolds ( in addition the raw water cooled elbows). The manifod could be plugged with mud or corrosion, or you could have a collapsed hose. On most engines, broken off pieces of a damaged raw water impeller can find themselves in troublesome spots.

Just a shot in the dark here but be sure you don't have the thermostat in upside down...don't ask me why I would ask this question.