water in the chimmney
Your chimney is not getting sufficiently warm enough to create a sufficient draft capable of removing the water vapor before it can condense.
Our chimmney was creating its own rain cloud.
We completely sealed the exposed cinder blocks with Thouro-seal (sp?) This was a dry material we mixed with water and painted on the blocks.
This prevented moisture from being pulled into the chimmney.
Solved our problem.
Hope this helps