It is possible that the leak is above the fireplace stack... in the direction of the peak... that a tiny leak is permitting water to enter and the water then is working its way down the inside of the roof (most of the time it will follow a rafter) 'till it gets to the fireplace stack where it hits the stack and drips down.

Your best bet is to bring in a local reputable roof contractor and let them asses and repair the problem. The longer you let this continue, depending on folks that have so far been unable to figure out what's going on, the worse the water damage will be.

Talk to the neighbors, check out the local phone directory, [www.angieslist.com] and local home service oriented web sites and get someone that understands roof systems on the problems ASAP