I wrote an article about what makes an engine overheat, and how to find it. It will run you through a little 10 minute test that will get you pointed in the right direction.
Usually when this happens, though, it is a head gasket...
Anyways, here is a link to the article. I know it has helped alot of people as it is ranked #1 on the site (a popular one), as well as Google.
Let me know what you find, and we can go from there if you still need help.