1997 Volvo 960 over haeting

My volvo960 was doing great but 5days ago began running hot! Steam began coming out the anti freeze reservior signaling overheating plus when I look at temp. Gauge it was buried on red. The car was down about a half gallon of anti freeze the garage tech replaced the coolant to its proper level and I experienced no more problem until 3days ago. The car began running hot once more and while it was running hot I cut on the heat and the temp. Gauge immediately went back normal. Since driving with heater running the car has not ran hot could it be a thermostate problem or more serious developement? Thank you for your time and coming advice