Temperature gauge suddenly climbed while at idle. Temp maxed out based on the gauge, to the point where I thought the truck should be steaming. Chime went off a couple of times but no lights came on. Got home and topped up the radiator. Took it into the shop and they pressure tested and all ok. Let it idle for 25 minutes and no overheating. I took it home and it behaved the same way. Gauge climbed to max but this time I got a chime and the Check Gages light came on. Any thoughts on what might be the cause?
2004 Didge Ram Diesel 4x4

Bob

 

Sticking thermostat?

Was it actually low on coolant? If it was, where did it go?

 

Down but not significant. I am leaning towards a temperature sensor or thermostat. It would not be a bad idea to replace both anyways.