I drove 10 miles the other day and all was fine. On the way back, my temp gauge went from mid to pegged all the way.
I put in a new thermostat and all was fine, midway on the gauge, for a few miles, then it began to creep up and pegged a few miles later!
I parked it and came back about 90 minutes later and turned it on an it was immediately at the halfway mark.
I got home today at 3pm and started the car at 8.15pm and the gauge was registering - see picture. The engine was COLD.
So with the new thermostat doing the same thing as the old thermostat, what is going on? Is there a sensor that should be replaced?
Thank you!

2003 STS