JimMc & Dark4ng3l:
Yep! You were absolutely correct!
I originally unlocked the Duron using a conductive ink pen. I thought of this as a possible cause too, but ? because of the conductive ink used originally ? I wasn't convinced that this was it. But, following your advice, I took the Duron to work and checked it under a higher power microscope. Sure enough, I didn't get a couple of the bridges covered well enough. So, while it worked initially, it didn't hold up in the long term.
Thanks for the tip.
OB