The first step in diagnosing a cooling system is checking the coolant/water mix. Is it clean? Is it green? Make sure it's not rust-colored or that it has oil floating in it. While you've got the cap off, check the radiator cores for blockage. Check out the article "Cool Her Down" (Sept. '05) if you aren't sure what that looks like. If the radiator core is blocked, run a flush through it. If there is buildup in the radiator, chances are the engine has blockages too. Make sure you've got the correct 50/50 water/antifreeze mixture.