Your thermostat is stuck open. It should stay halfway, no matter what vehicle speed, engine speed, heater status, external temperature, etc. The cooling at cruising speed is due to the coolant flowing to the radiator when it shouldn't be.
Mine stuck open a couple months ago. I got a new assembly from PelicanParts for $19, but I see now they are $31. Also, buy new upper/lower radiator hoses while you're at it, like I should have
It's quite easy to change, just four little bolts and two hose clamps. The worst part by far is draining/changing the coolant, it such a messy process.
Good Luck!