Check your headgasket by doing a compression test....
http://www.nicoclub.com/articles.php?id=159838
If your head gasket checks out okay, then chances are your radiator is clogged with calcium deposits. You'll have to have it chemically cleaned at a radiator shop or you can go & buy a new one. It's cheaper to have it chemically cleaned but you just need to make sure the radiator shop does a good job putting it back together after they cleaned it.
Anyhow, here's a link to how your cooling system works....
http://auto.howstuffworks.com/cooling-system.htm
Hope this helps..
ID