After chasing around after high engine temp problems, and replacing senders, thermostats,
wiring, etc.. on more than one engine, I have found that most of the time the problem was with the areas mentioned by other posters with poor
coolant flow being #1.
The wiring has nothing to do with controlling engine temp. The fact that your
coolant flow seems adequate at low
RPM, but not at higher RPM indicates
strongly that you have a flow problem. The first and easiest things to check are the hoses (which can look fine but are collapsed internally), strainers, and your
pump impeller(s). Dive under the
boat with a mask on and make sure your intake isn't clogged with weeds.
If all that looks OK, then you need to get after the harder to fix stuff. Clogged
heat exchanger, mixing
elbow, and the
cooling passages inside the engine.