[CWizard]

Ray - I agree with Tom. It's probably air getting into the exhaust, and from what you said, probably from a loose collector flange or a blown collector gasket. Because of heat and stress cycling, those bolts get brittle pretty fast and are easy to break. Some people have reported good luck with stainless steel bolts for this application. I had the same problem as you and replaced my bolts with grade 8 bolts with double nuts. So far, so good.
Hope this helps.