so, after getting nice, warmed up, and smooth running, the dashboard temperature gauge drops and pegs on cold...4-5 minutes later it comes back up to normal...then drops again after a half hour of running...then back up...the only consistency is that it's damp here in the pacific northwest...does not seem to effect anything except my nerves...i want to make sure the 6.0 doesn't get too hot, so not feeling i can trust the gauge....any ideas about why?

 

Could be the temp sensor, its connector, the wiring, or the instrument cluster.

Start w/ inspecting the sensor, its connector, and as much of the wiring harness as you can access.

See the "page 9" pic (ECT sensor):

Powerstroke 6.0L Technical Intro Manual