Temp guage goes to max (right) when engine is cold.

I have a 2001 Dodge Ram 1500 w/ 5.2 L engine. Ran fine until a cold spell hit. Hard difficult time starting truck, check eng. light came on, and temp. guage was pinned to the far right> An idiot light "check guages" came on. The truck was still cold, not even at operating temp, yet guage suggests eng. was hot (not the cae). I let it run for an hour put High Octane gas with dry gas and F/I cleaner. Although this resolved cold starts, eng. light still on and gas cap is tight too!. The main problem I need help with is figuring out why I was able to remove radiator cap with engine running for over an hour. no pressure or overflow, no coolant leaks. Temp guage fluctuates affecting idle and throttle response, but low end torque is lost when guage max out to right (pinned) even when eng is cold from over night. Does not matter if key is in acc or "on" w runing eng. Not stuck in position either. When veh if off guage is at O zero (left). Anyone els have this issue?