I had a weird problem on my little Volvo
It would run fine for hours and days. Then suddenly overheat. Raw water cooled but that may not matter to the solution.
This went on for four years. Drove me nutz and nearly ruined my sailing fun.
An old grizzled salt
finally came to the rescue
I had a fitting that required a copper compression
washer. It was in the line from the transmission
to the engine. When they rebuilt the engine they omitted the copper washer and used silicone instead.
Normally it would work fine, but once in a while it would develop an air leak and stop pumping. Then self heal. Then fail again.
I didn't even know I didn't have the right thing since I had not seen it put together correctly and I'm a novice
on much of this (but gaining experience daily.)
Replacing the copper washer was all it took.
Insidious to be sure.
I would go back over all my fittings to assure that all is according to Hoyle.
Almost every issue I have had with the engine can be traced back to the rebuild
and them taking some short cut. Maddening.
I once had a boss who would always say ..... "If there is a new problem find out the last place someone worked and go see what they messed up."