heat gauge problems

I have a 95 bronco xl , about 2 weeks ago at work i started my vehicle to let it warm up while i went back inside. i came out to see the heat gauge was maxed in the hot position. I poped the hood and checked the engine and could tell the engine wasnt that hot and i could put my hand on the rad still so it couldnt have been that hot either. I had a new thermostat put in and it worked fine for the past 2 weeks. today i drove home from work and the heat gauge worked fine all the way home. i got gas across the street from home and when i started my vehicle again the gauge spiked to the hot side again. when i shut the vehicle off the gauge does not go below half way , even after its been sitting for hours and has to be cool . any sugestions as to what the problem might be?