A mate at work had almost exactly the same issue as you on his NA XR6, very similar symptoms. After taking it to a few workshops with no results (including Knox Ford) I pulled the rocker cover off to find it had skipped a tooth, the top of the tensioner/chain guide was chipped off too. I set the timing again and its been fine ever since. He should replace the guide but he hasn't, so far so good lol. NA engines are cheap! I would be surprised if that's not it!