My 2000 Dodge Durango Sport's temp gauge keeps moving up and down. When the truck is sitting, it goes up, never into the red, but close. Then works it's way back down. Got a new t/stat, same problem. 3-4 wks ago I added 50/50 Antifreeze, and it is still full. Any suggestions on what to try next?

 

Repost in the first generation Durango section as this is no mans land,
FF