"hpeer" wrote in message
...
This particular water pump is on a little shaft on the back of the
engine. I suppose the pump could be failing to turn intermittently, I had
not considered that possibility. Seems kinda far fetched, but then again
this whole problem feels weird.

I have had this trouble on a petrol engine in my 22' motorboat and the cause
lay in the rubber impeller in the pump.
It is easy to examine the impeller and, seeing no wear on the ends of the
lobes, to conclude that it is Ok for further service.
However after some time the rubber goes soft and this means that the lobes
cannot spring back quick enough after passing the cam to allow water to come
in through the inlet.
You may get away with a soft impeller at low speeds but when you increase
revs you will lose suction..
Also, with a soft impeller the pump becomes more sensitive to back pressure,
which in extreme cases can allow back leakage past the ends of the lobes.
Your system seems likely to create quite a bit of back pressure especially
if the taps you have in the system are not gate valves or ball valves which
allow full bore passage through them.
You need this sorted out quickly because you are salt water cooled and every
time you get temperatures over 140 degrees deposits will come out of
solution and ultimately cause severe problems.