Stosh, if you've ever been in the mud, it's probably what's causing it. One of our members had similar problems, and the dealer said "replace the radiator".

We spent a couple hours with the hose after releasing most of the radiator fasteners and rinsed out a lot of mud and leaves that weren't really that visible. All his overheating problems were gone. It doesn't take much mud and crap to block enough air flow to overheat.