Engine running hot.

2003 Mitsubishi Eclipse:81,000 miles:
**Every time I come to a stop, my car slowly starts to overheat and the temperature gauge rises all the way past the H. BUT, when I accelerate the gauge goes back down to normal. I notice it a lot when im at a drive-thru or at a stoplight, but nothing can make it go back down except accelerating. I checked the water level and everything is fine. I don't know a whole lot about cars but I know that it cant be good when your car runs hot. I'd appreciate the help so much! Thanks! :)