random fl;uxuations at temp gauge?

ive got this rerally odd problem...well odd to me ive never had this happen before... i get random fluctation in my temp gauge...truck runs great other then that....something ive noticed...if im running my headlights or something like that and i switch them on and off or rev the engine sometimes it wil drop back down to normal...truck runs just under half normaly...but when the gauge freaks it reads 3/4 and rises fast (5-10min from reg reading)....ecu seems to be fine and didnt find much on this...as well i didnt want to just throw parts at it...any ideas?

I'm just wonderin' if you have ALL the air bled out of your cooling system...these trucks are a little stubborn with that....

Have you tried replacing the connector at the gauge sender with a new 18-22ga bullet connector? My connection was bad and did screwy things, that fixed my problem. If that's not it, I would replace the sender itself as well, should be cheap.