engine overheating - almost

I have a '00 Exlporer Sport...and the temperature guage seems to be quite eratic. I only drive about 15 minutes or so to and from work, but in that time the guage clims to almost overheat, then drops and hovers around 60%-80% or so. It does drop when I stop at lights, and clims while I'm driving....I also haven't heard and obscene noises that would make me think it was the fan or water pump. I'm not very car savy, but would my problem be a circulation of collant/antifreeze? I haven't seen any leaks at all....what would I need to check?

thanks for your help!

Your thermostat is not working, replace it and car should go back to the normal tempature.

thank you...I'll see how that does