Yesterday, I drove about 100 miles. 50 mile trips non stop. It was on the freeway, and my average speed was around 75 to 80, peaking at 90, so my engine was really moving. I notice coming home that my temp is really high, like well over 200. With the new thermostat I got in, the car never went over 200. I got home and let it sit for about a half and hour, and then drove to the bank, which was only a mile away. It was still over 200. Probably around 205 or 210. Then I let it sit for about 4 hours. It hovered around 200. Today, I drove it for a little while, and it was fine.

I don't know why it got so hot. Could it be in the radiator? Or the coolant?

Is your coolant level good becuase if its not sometimes it wont have enough pressure to open the therm. Also check your radiator.. Maybe its damaged look at the front see if any of the wings a folded check also for an abundance of bugs..... I had that prob with the bugs and I had to clean out my radiator.. Uhmm and maybe your water pump is going bad Mine went bad a few months back and it wasnt a good thing... It might be a good idea for you to get that changed or do it yourself its not to difficult. Anyways good luck and dont overheat your engine....

The air was let all out when it was changed. It has been in for a few weeks now, and it'* been fine ever since this incident.

But I will check the radiator.