The problem you're describing sounds like a capacity problem. I had a Honda that did something very similar. If I kept the car under 65 mph it was fine, over 65 it would slowly heat up.

I flushed the cooling system, but eventually had to replace the radiator, which fixed the problem immediately. So, I assume the radiator was clogged, which reduced its cooling capacity. Good luck, I hope this helped,

Don

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2008 ML350
2007 Honda Element
2007 Honda Accord
2009 Honda Civic