Hello,
If the thermostat was sticking, it would have shown up during your initial check; furthermore, with the fans coming on at intervals, you already know the coolant is moving.
Usually a overheat at a cruise is attributed to exhaust gases leaking into the cooling system, or a water pump failing at higher RPM's {real common on VW's}
To check for a exhaust gases, a tool that uses a special liquid is inserted into the coolant fill. The CO in the cooling system will turn the color of the fluid.
What is it that you are driving. It did not show.
Scott.
The first thing I would do if you verified that the engine does not overheat at an idle and the fans cycle on and off as you see the temp gage drop and rise, would be preform an block test {the fluid} As you speed up the engine, you create higher pressure in the cylinders and if you have a slight crack in the head or a failing gasket {can be caused from lack of service of the cooling system over time}
If you had a clogged radiator or cooling system, it would be noticed at an idle {less air flow}
Verify the no overheat at an idle, and then I would reccommend getting the block test.
ASE Certified Technician
Ase certified, 24 years, bumper to bumper all makes