A year ago,I had the same thing happen to mine. It turned out to be a "steam pocket",somewhere in the block. Luckily,it over-heated about a 1/4 mile from a mechanic friend's place. He started it,and ran water,(from a hose),through the radiator,until the engine cooled off enough to safely remove the cap. He then stuck the hose into the radiator,and held the throttle at about 2,ooo R.P.M.,while squeezing the upper radiator hose,to "burp" the system,of trapped air. After the 20-minute procedure,the "Stat" was drilled with a small hole,to let air escape,to alleviate the problem. It's been running on the "cool" side,ever since. Hope this helps.
