Had the same problem.
Scangauge water temp would creep up towards 200* on highway cruise, temp gauge would not move because I pay attention to my gauges - but downshifting & flooring the car would bring temps back down to the 170*-180* range.
I put a BIG Koyo Radiator in & STILL had the same problem.
Then I changed the Crucial Racing thermostat in my car for a stocker & problem was Solved.
Now HW cruise @ over 50 mph will have coolant temps in the 120* range which is not good - so I have to put a piece of cardboard over the radiator ice road trucker style to keep coolant temps up.