Sounds like you are getting gases back through the threads. I would try another breechplug but in the meantime (and maybe afterwards, try Sabotloader's method of taping the plug with teflon tape. A single damp (with water or Windex w/vinegar) patch, both sides followed by one or two dry patches should be enough to clear the worst of it.

You aren't leaving plug grease on the forward end of the plug, are you? Too much grease in the threads can push forward into the ignition area as the plug is tightenedand cause grief. There is no way you should need to fully clean after each shot.