I have seen a problem with the radiator getting plugged up and not flowing properly. It happened to me and two friends. Pull the rad and take it to a good shop for a flow test and have them flush it. Also, if you mix anti freeze, the old stlye green stuff with the new Dex2, it will gum up your cooling system. Try opening the radiator drain, because those of us that have had clogs, it seems to be at the lower portion of the rad and it doesn't drain from the petcock.