The lack of replies is because topics like this have been discussed before. Look at the how-to articles above on how to bleed the cooling system properly and and a replacement thermostat. I'm guessing the spring portion itself on the thermostat is stuck open, causing too much flow and never letting it get warm enough while driving down the highway. Since you're not moving as fast when off the highway, the radiator is allowed to heat up again. If you're X has around 100k miles, I always suggest a complete cooling system overhaul.

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