To reiterate * ditch the Bosch plugs!
I LOVE Bosch plugs for european applications, they last forever, but they just plain suck in these things, there was a poor application match up. if you use those Bosch plugs, you will be back in the same boat before long. Stick with the collective wisdom get the Beru/Motorcraft plugs.
Look REAL carefully for score marks on the cylinder walls and strongly consider the advice of replacing the rings on that hole.. if there's a little piece of plug stuck in the upper ring grove, that ring won't flex and you'll loose compression ( run like crap and burn oil )