Do you have oil and water temp gauges? If so what are the actual temp numbers when this happens?

It may be worth checking the radiator for bubbles or gas while the engines running just to rule out the possibility of a head gasket leak. I've had one in my EG before and it caused this issue almost exactly. Combustion gasses entering the cooling system and causing it to overheat on the highway.

If that all checks out, you might consider replacing your thermostat.