Tohatsu/Merc 30hp EFI water pressure/overhear issues - at a loss, S/N 0R341278

Hoping someone can help. Since this 2010 Mercury is really a Tohatsu, I'll try this forum instead of Merc (tried it there but got no hits).

Issue: Running boat in water causes water pressure from tell tale to drop off and become weak. Motor will eventually heat up. Had one overheat alarm (the first time). If you shut motor down, water pressure returns and appears normal. Is also normal in neutral with higher RPMs as well, issue only seems to develop when underway. I made sure the pickup screens are not blocked or tubes clogged, and made sure intake screens were on the proper way.

I suspected water pump cavitation. I have entirely replaced the water pump including upper and lower housings, gaskets, wear plate, cup, impeller and key. I also added the rubber centrifugal slinger on shaft that is shown in parts diagrams (part is NLA but I found a grommet that worked). I also replaced the thermostat and flushed the motor with it out. Motor still does it.

When running at dock, water stream heats up to approx 95-100 degrees and doesn't seem to get hotter. I blew out all the water passages as best I understood them and don't believe anything is clogged. If there is, it's a passage I'm not aware of.

Could this be a leaking exhaust gasket? Could exhaust be entering the water jacket and causing pressure issues? Would there be other symptoms I could look for? It's frustrating to test, as you have to have the boat in the water and underway for it to do it. Works completely fine in the bucket, at least I think it is. I can't find a mechanic that has time to look at it now, so trying to get it solved on my own. I'm handy, but feel like I'm out of ideas.

Thanks.

I have never seen a base gasket leak do something like that. It's possible that your jack height is too high.
I doubt blockage would cause this problem, as you would also see it in the test tank.
Is this a long shaft? Is it stored outside in winter? If so, the water pipe could be crushed by water freezing in the driveshaft housing, unless a drain hole is bored near the bottom. In that case, the problem would only show up at higher speeds.

Thanks. Not stored outside. Stored in a garage all winter never seeing temps below 45°. I checked water tube though and didn’t see any issues with it. Also motor is in same location it was since new and this just started this season. It is a long shaft too. I have it to a Mercury service dealer this morning that actually has time to work on it. It had an occasional stall too so he’s going to look at that.

Did you find the cause?