So noticed it last week, temp gage was bouncing around, Thought the sender was bad replaced that, Then my alarm started going off, So as long as I kept it under 3k rpms not as bad so I limped it in, So thinking I may have chunked an impeller, I ordered a thermostat kit and also another impeller, Impeller looked fine switched it anyways, As so did the thermostat. So took every hose off of everything and there's plenty of water flow going everywhere, Ran the boat without a thermostat and it runs perfectly fine temps don't really move, once warm, What am I missing here where should I look, ? Also installed a strainer So I can make sure there's water flow, But also my water pressure went down to like 2 p s I,

Take the hoses off the large heat exchanger. Flush backwards with hose. you should be able to look with a mirror inside to see if you have and weed blockage.

Most likely an old school 525sc that is raw water cooled. No heat exchanger.

Have you checked for "bravoitis" ?

Brand new gimble assembly,

...

I didn't back flush to oil cooler though, When I take the hose going to the thermostat off, has really great flow, why i didn't think I needed to do it,

Reach inside the water intake side of the oil cooler and check for any debris

What gaskets are on the end caps of that oil cooler,?