I am having a similar issue with my 197 f150 4x4 4.6L. I was having gauge issues (Oil pressure and voltage). I swapped out the oil pressure switch and found I had a bad ground. Anyways, upon restart those two were fixed but the temp gauge was pegged. UGH!
I replaced both the temp switch and the coolant temp switch, still not fixed. If I pull the plug the gauge reads Cold., If I short the temp connector it reads hot. The factory service manual says it is a switch issue... tried 2 new ones no luck.