Truck temp climbs while not moving.

Under normal driving, the temps stay normal. But when I hit a stop light the temp starts to climb to 230ish degrees. Then goes back down under normal driving.

Replaced the Tstat with the same kind LS 160 degree tstat, and filled it back up with coolant (about 1 gallon of 50/50) and let it idle to see what it would do. After it started getting to normal operating temps, it slowly started climbing again. Shut the truck off and came inside to type this.

Any ideas?