[67JET]

This may sound odd but when i have issues such as yours and can't find an obvious problem i change out all the plugs first. I had a dealership tell me once that i had a bad motor with no compression and that is why it wouldn't run. After talking with my Dad and calling him crazy a few times i called the dealership and told them to put new plugs in it and try it again. Then they called me crazy a few times and said if your paying we will do it (i was out of town and couldn't do it myself). They called me back and said it was fixed and didn't charge me labor just for the plugs. My experience was just like yours - run great-run bad-stop running. Maybe you can just check them but you can buy a cheap set of plugs and know for sure.