I need HELP! At the end of my rope! We are living in a 1959 brick ranch in NE Ohio which has a plastered interior and Celotex then brick, no insulation, on the bedroom and living room walls, and the bath and kitchen having been remodeled and now have insulation. We installed a new roof in 2014, new windows in 1998. The attic was newly insulated in 2020 with blown insulation, baffles installed in the rafters, and a solar fan installed. Also, have a ridge vent and soffit vents. We have Geothermal heating and cooling system and experience a lot of moisture only in the winter and this is when the mold and mildew appear on the ceiling/wall areas of the outside walls of the bedrooms. It is only in the winter this happens. You can almost see a dark area about a foot in from the walls along the ceiling that appears ‘darker’ than the rest. I apply a mildicide to remove it but it comes back. The plaster is now to the point that the paint is peeling off the ceiling. The walls are currently wallpapered. I’ve approached everyone I can possibly think of with this problem without any solution. Short of selling the home we love, I don’t know what else to do. We are currently thinking about removing the plastered walls and replacing it with drywall after insulating between the Celotex and brick. 
Can someone please, please help me with a solution? We would be eternally grateful for any help with our situation. 
Thank you in advance.