I dont agree that the Montero was not designed with speed greater than 55 in mind. I know that has been repeated lots of times, but that does not make it true.

On my Gen1 4 4 cly, when running only the cooler in the radiator I found that punching it on the highway op long grades did get the temp lite on. That was with a motor that makes 240 ft lbs of torque at highway speeds.

I also had a smaller converter. Once I hooked up the auxiliary cooler the unit never overheated. IT still ran too hot..

Factors I would look at.

Taller tires, past 31's add to the problem.

A temp switch that's coming on too early. I never hurts to check the pan temp.

As Eddy pointed out, a partially clogged cooler, either internally or externally.. Perhaps both.

What size tires are you running?

A larger cooler never hurts but it good to make sure you dont have some other problem.

Kevin