What is this in? Regardless, if the temperature gauge is operating properly then try the following fix. Go out and get a thermostat for a 440 in a motor home. This will open at a lower temp and keep the car from initially heating up. I would also suggest you run a radiator flush. Not for the radiator, but the block. The engine cooling ports could be clogged or semi clogged. If you don't have a fan shroud, invest.

If it heats up when driving on the highway this screams of pour cooling circulation. It would just plain overheat if the thermostat wasn't opening.

Good luck