The thermostat suggestion was probably a reasonable suggestion, since it's one of the cheapest things you can try to fix; but thermostats almost always "fail open" (they're supposedly designed that way) so they almost never cause an overheat problem.

I don't have "the book" on your model, but suspect it does have an electric fan, so that needs to be checked.

You've indicated there are no symptoms of failed head gaskets. If you haven't looked already, you should be able to see water moving with the radiator cap off, which will give some indication of whether the water pump is working. (don't open while the engine is hot, but warm it up some with the cap off if there's any question about whether there's motion there) Sometimes you can feel the "movement" by squeezing one of the larger hoses, so you might try that before opening the system.

A fairly common cause of overheat in "newer" cars (yours qualifies) is one of those many vacuum hoses that ties into the emission control systems. If one of these comes off, or leaks in a "major way" it can cause a lean mixture that can cause overheating. Since you indicate that the overheat occurs at "cruising speed" when the mixture would normally be fairly lean, this is a good possibility.

Vacuum from the carburetor base is also sometimes used to operate brake boost, heat and vent actuators, and such; and a break in the diaphragm of any of these can cause a lean-out that can cause overheating.

Unlikely possibilities:

a dragging brake, possibly due to a sticky parking brake cable or linkage.

If you have an automatic transmission, a slipping clutch or malfunctioning "governor" can put rather high loads on the engine, which could cause an overheat. (The power "wasted" in the transmission forces the engine to produce higher than normal power output at a given speed - the max torque isn't really much higher.) Extreme clutch slippage with a manual transmission would cause the same thing, but usually you smell the smoke before the engine gets too hot.

Have faith - there is a reason, and you will find it.

John