If you are running a newer, more efficient furnace into an old unlined masonry chimney it will be a problem sooner or later. Usually this shows up sooner than eight years.The stack temp is too low to evaporate the moisture inside. If removing the cap don't help you should have a metal liner installed or eventually the whole outside of the masonry will turn to snot. That is why water heaters vented in an old masonry chimney is not recomended. It will do this even in a clay lined chimney but it takes a few more years. I've relined many with stainless steel liners and it has worked every time. Cheap to do but expensive and messy if you wait till the chimney is too far gone because the plaster or drywall or anything touching the masonry on the floors above will soak this snot up and will stink forever. Smells like coal smoke.