Recently bought a 2007 Ford Taurus from my parents and while my husband was driving to where we live (4 states away) he had to pull over multiple times because the temperature gauge said the engine was too hot. My dad topped off the coolant before my husband picked up the car. When my husband looked under the hood, the coolant was low, so he refilled it and kept going after the engine cooled down. Same thing happened, he again refilled coolant, but this time turned off the AC. No problems, no leaks. When I drove the car to work today, AC was off and I had no problems. What could be causing this issue?