Engine over heating

I have a '02 honda civic and it has 65,000 miles on it. Recently, when im driving the engine thermostat gets to 1/3 of the way up and when I idle at a light or in park the gauge gets to 2/3 of the way up to the top. If I start to drive it agian at steady speed, then the thermostat goes down back to where it originally was. However, the longer I drive it or if I drive it for more than 10 mins on the highway, the needle will stay at 2/3 to the top. If I drive it for a long period of time, (over 30 mins) when I idle the gauge will get all the way to hot. Also when this is happening, my air conditioning blows warm air until the needle gets back down to the middle. I was wondering what could possibly be causing this problem.