We have moved over the past 2 months and our previous house is for sale. Over the past 2 months potential buyers have complained about a horrible smell as they proceed up stairs to a finished attic (1000 ft of living space). I have isolated the smell to a back room with ceiling duct work and vents. This room is the furtherest from the air unit. There are no windows to this room. The smell seems to be coming out of the ceiling vents when the unit is on. Based on the length of time and the inconsistency of the smell I have eliminated a dead animal as the source. I have also had gas company check to be sure there are no leaks. I think it may be associated with rain because that is when it seems most prevalent. This at the far end of the duct work closest to the back of the house where the outside wall vent to the outdoors is located. Any help/ideas would be appreciated.

You may have a mold situation. The only way to find out is to look for it. Mold can get into duct work so some might have to be taken apart to have a look see. Mold can really mess up a sale. I suggest you get someone on it right away.

Would the mold smell come and go? Sometimes with the air on the smell is there, sometimes it's not.

Mold smell can come and go depending upon if the area is moist(wet) or not. When mold is moist it is growing and giving off the smell when dry it has been robbed of it moisture source and does not grow. The only way to figure out the situation is to take action. It could be something different from mold also like a cracked/leaking plumbing vent etc.