I agree w/ JMC, and would strongly suggest testing your t-stat (you may say to yourself; "I'll just replace it!", but you still need to test the old one to see if it may have caused the problem), and trying what he suggested for the water pump. It sounds to me like a failing water pump, but don't just go out and buy one without further testing. Water pumps usually leak before they fail, unless the failure is the impeller fins becoming worn. I know there were some 351's with plastic impellers that had that issue, but I don't know about the mod engines.