Leaking head gasket

My head gasket was leaking, car was having white exhaust, and a rough engine and leaking coolant.

Okay, so I had a mechanic put in the good stuff, the head gasket sealant for the head gasket. I made sure it was the right sealant for the Nissan and the best I could find. This was three weeks ago. Immediately the leaks stopped, (car was leaking coolant not oil) no white exhaust, no coolant leaking.

Radiator is still full to the top with the coolant that was put in there three weeks ago.

(Yes, the mechanic gave it an oil change and flushed the radiator before adding the sealant.)

I only ran it to work and back about twenty minutes, with the heater on full, for circulation, ran great for two days. Then it started overheating after driving for twenty minutes.

After about twenty minutes of driving, When I stop at a red light, or slow down, or hit traffic, the temperature goes up to just below the danger line or the last quarter of the inside temperature gauge.

Once the car starts to move again, the temperature will start to slowly go back down again.

Up to twenty minutes the temperature holds perfectly at the half way mark with freeway driving. And I have to say, other than the over heating, the car has never run better. Noises from the heater are gone, engine is quieter, things like that. Transmission is strong, radiator is not leaking.

The mechanic took another look at it, but could not find anything wrong. I do not think he looked that hard.

I have heard that sometimes if the thermostat is not removed while adding the sealant to the car, the thermostat could get stuck.

So I started to think, if the thermostat is stuck maybe when the car really starts to heat up at about twenty minutes, then that might be what is causing it over heat?

Is it worth replacing the thermostat or is it off to the salvage yard?

Oh, I should also mention that before the head gasket broke down the thermostat had a little leak. It was not bad just something I knew I had to get fixed eventually. The coolant only needed to be topped off every three hundred miles or so and then only quarter of a quart.