rover overheating im at a loss

I own a 1996 4.6 hse range rover. It runs hot above 65 mph (highway speeds) and only sometimes at idle, and gets hot faster with the a/c on. i have replaced the radiator, hoses, coolant and thermostat also there are no leaks. A land rover mechanic told me the fan and clutch is good. Before I acquired the rover someone replaced the heads , water pump (it runs very smooth)installed a mechanical temp gage (when dash gage is in the red zone with the light coming on occasionally, the mechanical gage is reading around 207 degrees) and also wired the electric fans to come on when the rover ignition is switched on (runs all the time).sorry I don’t know much of its history. My question is what have I missed? I am at a loss on making this rover run cool.

sounds like the possibility of warped cylinder heads, I would run a cooling system pressure test, even a carbon test to find if you have presence of hydrocarbon in the coolant.
after that I would do a coolant test flow to make sure the systen is free of restrictions.. all of the tests can be performed by a radiator shop.
if it turns out you have warped heads, they have to come off and be milled to specs.

or it could be bad battery and engine grounds.