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You don't mention your location.... if in a heating climate, usually water marks are sign of warm moist interior air condensing on a cold drywall ceiling- the "first condensing surface"; http://www.buildingscience.com/docum...bout-diffusion
You may have oil-based paint with cold drywall due to lack of insulation behind/above the condensing area. The inlet from soffit may be moving air past that area, check for continuous soffit venting for each rafter bay and figure your NFVA for your attic; http://www.airvent.com/homeowner/pro...it-specs.shtml
Pictures of the existing intake vents (from the ground) will help us to figure your venting for you, is there a poly sheet next to the ceiling drywall on the attic side?