Temperature Gage

My temperature gage normally stays put when driving at the middle of the dial. In fact almost always. However. Once I need to slow - such as bumper-to-bumper traffic - the gage has started to rise up, indicating an overheating. It usually stops at about the 3/4 level - it never reached the RED line at the top of the gage. No other indicator lights go on. Once I resume a speed of any consistency - 15 MPH or 60 MPH is irrelevant - the needle noticeably and rapidly falls back down the mid point of the dial, which was where it was before the crawl in traffic began. I'd if you could give me a direction on what you think the problem is I'd be grateful. Thank you. Nick