1991 Jetta's thermostat
 

My car has overheated a couple times and is at the point where it might overheat again. When I see the the thermostat go up I crank up the heat causing the thermostat to back to normal.

It's 25c outside where I live, I really don't want to put my heat to full blast in this weather.

Any ideas on what's wrong?

Thanks.