overheating or not?

I have a 2003 Hyundai Elantra with 150,000+ miles. The problem is the
temperature guage reads hot, at speeds below 50 mph and the air conditioner blows hot air in the summer or cold air in the winter when this happens. This is not an everyday problem, maybe once or twice a week.
I have put three new thermostats in over the last 15 months and this
"fixes" the problem temporally, the last on is only 1 month old. there is no water boiling out as the radiator is still full of anti-freeze. I have also
checked to see if the fans are running and they are. Any suggestions
would be apprecited.