73itb: I had very similar symptoms last year. Overheating, and loss of water - no oil in the water, no water in the oil, no white smoke. Presure test of cooling system showed a good system. Test of the radiator cap showed that it released at the right pressure, but did not reseat properly, so I replaced it with a new cap. When that did not fix the problem, I ran a compression check and found one cylinder significantly lower than the others. With that new information, we tried one last check. With the engine cold and the cooling system filled, we started the engine with the radiator cap off. The water shot up out of the radiator about two feet! Turned out that the head gasket was broken between the cylinder and a water passage, which presurized the cooling system when the cylinder fired, forcing the water out of the system through the overflow tank. Hope this helps.