temperature gauge pegging to 260 on cold start

I have a 2002 Dodge Ram 2500. I was on the road and the check gauge light came on and my battery gauge was sitting on zero. My husband told me to take the truck to O’Reilly’s and have them test the battery and they told me it was low. I asked if it could be the alternator and they tested it and said the alternator was bad. I bought as alternator and we install it. Something still didn’t seem right so we had the battery tested again and it had a dead cell so we replaced the battery. Now I have a problem when I start the truck from cold the check gauge light come on and my temperature gauge pegs to 260 and after a few minutes of running it drops back down to zero until I drive it awhile and then it slowly rises as the engine heats and it goes to normal operating temperature, but never hot. We replaced the engine coolant sensor and it still does it from cold, but if I drive it until it reaches normal operating temperature and then leave it to go in the store, it doesn’t peg out when I start it again. It only does it from a cold start. Any suggestions would be appreciated, I’m lost.