1991 Grand prix temperature gauge problem

Well i was driving my car the other day and the tempeture gauge went all the way to the top all of a sudden past the hottest tempature on the dash. So i let the car cool down so i could check the radiator was filled properly. The gauge never went down as it sat there, it just stays there all the time. I initally thought it might be a thermostat but when i accelerate while driving it the gauge goes back to normal as long as i'm accelerating then goes back up to the top as soon as i take my foot off of the gas. Anyone have any idea what this might be that's causing this? The radiator was full. the overfill wasn't so i filled that. I'm just wondering what the problem might be

oh forgot to mention it's a 3.1 if anyone is wondering

Are your cooling fans running at all?