I had a problem similar to this. The motor had no thermostats, showed water pressure, but almost burned one CMI header down at top end. I tried everything I could think of. The start of the next season I had no water movement at all. The sea pump impeller was replaced, and the rubber tips of the old one were pitted. Problem went away!

You might look at the intake ends of your coolers there may be something obstructing the flow.

You could have someone observe your tailpipes at WOT to check the flow. I had one header running dry, but only at top end.

These are just some additional thoughts.

HERE`S THE ANSWER:
 

I had the SAME EXACT problem earlier this year. I tried t-stats among other things. No change.
Several people suggested my impeller was bad. I thought: no way, I just put a new one in this spring.
I pulled it anyway and found that the corners of each blade were damaged. I popped in a new impeller, no more overheating!!!!!!

I replace my impellers every 2 years as part of preventative maintenance. It's pretty easy once you get used to it. Its a real pain once they fall apart and you have to get all the pieces out.

The correct type of water hoses usually have a wire sandwiched in them to keep them from colapsing.

KAAMA
I've had 2 boats where the inner lining of the hose "delaminated". It would collapse to less than 1/2 normal size. The outside of the hose still felt solid because of the wire support in it. You could only tell by looking down the hose itself. The hoses were about 10 tears old though.
Gary