Recap: Recently, boat started overheating for approx. 2 to 4 minutes
after startup. Temp goes to 220+F then it suddenly drops back to 150F
and stays there, then all is fine until I get to my destination.

The obvious culprit (I thought) was a faulty, sticky thermostat. I
figured the thermostat simply does not open when it is supposed to (at
approx. 160F) but eventually it does and the water cooling happens.

Bought a new thermostat and replaced: no luck. Removed the
thermostat entirely, closed the housing and started the boat: needle
creeped up to 200+! Went home and tested both (old and new)
thermostat in hot water: both appear to open at around 160F. So, it
appears the problem is NOT the thermostat.

I also disconnected the hose from the transom to the thermostat
housing (the one drawing the water from the ocean). Started the boat
for a few seconds and got a good flow so apparently no clogged intake
either. I am assuming that water was pushed through by the impeller
so that would mean the impeller is also doing its job (although I have
actually not looked in there and impeller was replaced 2 months ago)..

Now what? I am not sure what else to check. The alternator belt
seems tight so I am also assuming the water pump is running at the
correct speed (but I don't really know).

In the manual, they list the typical symptoms of overheating (loose
belt, clogged intakes, bad hoses, defective thermostat, worn or
damaged stern drive water pump). Pretty much the stuff discussed
above but also a couple that I have no clue what they are talking
about:

a) Excessively advanced or retarded ignition timing (????)

b) Leakage of air or exhaust into the suction side of the stern water
pump (????)

I am not very mechanically inclined so I get lost quickly in the
jargon.
Anyone can offer suggestions, advices, comments, recommendations? I
am getting desperate! Problem is I have a cabin that is boat-access
only so no boat = stay home instead of enjoying a gorgeous weekend at
the cabin...

Andre