they put a engine together for me before and used head gaskets from engine rebuilders supply and charged me for fel-pro gaskets. only reason i knew is i blew the head gaskets with a 100 shot after he told me it was good to 150.
how many miles are on it? cats are 8 years 80,000 miles. last one i did we had in stock. they had issues with them and some trucks got extended coverage. when i read your orginal post i was hopeing it wasn't a cat since you only had the mis on one cylinder. don't drive it if you can help it as i have seen two engine failures because of this problem.
spray carb cleaner on the intake, almost sounds like an intake leaking. there is a redesigned gasket to correct this issue. have seen many do this, we have also had a problem with a few injectors but more offten than its the intake.