I have seen similar things with some port bottles. My first question
is: what type of corks are you using? my second question is: is it
possible that you left a wine residue on the top of the cork? Once
the sufite reacts off, any remaining wine on the outside surface of
the cork could certainly mold up if mold is in the air. Remedy
options 1) clean the corks and bottle tops with b-brite or something.
Possiblly use a light solution of bleach as a last resort 2) remove
the corks and replace them with new corks. Before removing the old
cork, use a 10% bleach solution to kill anything there. Let the
bleach stand wet for 10 minutes, then rinse well. This will prevent
contanimation into the new closure.
If you are really worried, option 2 is the most conservative, but 1 is
pretty easy and you can do them all at once.
BTW, how do you know that it is mold? Maybe it is mildew. Maybe it is
dried seepage past the cork? you could see the latter since it is a
clear glass bottle.

It could very well be mildew... They have a bit of a mildew smell to
them?? There really is no sepage... What I'm noticeing is along the
cork on its sides. The top of the corks are dry and appear fine. They
are the best real corks I could get from st. pats... Put in with an
Itialian floor corker.
I like your sugestion on useing bleach. I fill my bottles with a
small hose and it is possible I had wine residue along the neck of the
bottle and that is molding/mildewing?? Do you usually wipe the inside
of the necks down before you sink the corks?