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Ken,
In 1980 I built a house that sounds similar to yours, in that it too had 6" walls, 1" styrofoam. I wrapped the walls with 6 mil plastic, but since the fiberglass insulation in the ceiling was kraft faced, I didn't put plastic on the ceiling. I never had the mold problem which you describe.
You didn't put plastic on the ceiling over kraft faced insulation did you? That could be a problem. Is your heating system forced air of baseboard? Mine was forced air which I would think would give better air circulation in the house.
Are you sure it is mold? I live in a house now that gets dark lines along the ceiling joists near the outside walls, but it is like a build up of dirt, not mold. Plus it takes about 5 years to show. I blame it on the temperature differences in the winter at that location and the breathing of air through the drywall.
I don't have a solution for you, but I hope the above helps.
Good luck!
Dodgeman