How did the second dealer fix it? Did they just try to re-tap the threads or did they install a sleeve? I would venture a guess that that spark plug (if not some of the others too) is still not installed correctly and/or isn't gapped properly. Of course, now you're caught in the middle between the first dealer who fucked it up and the second who tried and apparently failed to fix it. I would keep whining at both of them until you get some satisfaction.