1998 Chevy Cavalier Intermitant overheating

My engine seems to be on the verge of overheating but it does not occur all the time. The temperature guage in the dash reads very high on its scale and sometimes it goes beyond the high end but it has never reached the red because I turn the car off. The last time it happened I pulled over and stopped the engine. I then checked the coolent reservoir under the hood which seemed to be boiling. I unscrewed the cap for a moment then when it seemed to want to overflow I screwed it back on. I parked the card and did not drive for a few days. The next time I did the guage seemed to run normal (in fact it was in the low end of the temperature side when I was on the highway). When I stopped at work and let it idle it inched back up to the very high end. Sometimes it runs completely normally. Does anyone know what might be going on?