Well, I can relate to this problem. Was having the same issue with mine, changed water pump, thermostats, still having overheating. Found out about the PRV valve. Took it out and cleaned it, replaced it. NO MORE PROBLEM!!! Worked like it originally did.. There was not much in the way of dirt, just a little sand, but enough to keep it from making the proper seal. Simple fix and made me more confidant in my motor..
Let us know what does it for you!!!