Thats really bizarre then. Out of curiosity more than anything else, I would ask him if he checked the block for warpage but it shouldn't be and he probably did. If the head checked out ok and you haven't overheated it (until your recent problems), it shouldn't be cracked. There is a possibility that you got a bad gasket but I haven't ever seen that with the good fel-pro gaskets. I'd be guessing at this point if I were to say what I thought it was. I'd recommend taking it back to that mechanic and letting him give it a once over.