Both the plugs that I had that were bad (autolites) had some internal problem as well. One was arcing up inside the metal part, to the body of the plug, and the other just wasn't consistently arcing anywhere.
All of the Delco's I've had that were "bad" (either my fault or not) either cracked off where the insulator meets the metal body, or the insulator was actually loose in the metal body.
Just things to look out for. The days of just taking a plug out of the box, and even just gapping, without inspecting the plug, appear to be long gone. Not sure about NGK's or Bosch stuff, but then again, I heard NGK is making Delco's plugs anyways.