Smokey smell upstairs

Evening folks,
I'm having an issue with a smokey smell upstairs when I have a fire in my fireplace. The top of the chimney is adjacent to a gable vent on the end of my house. I have noticed that you don't smell it until the upstairs heat comes on. The upstairs unit is in the attic. The return for the upstairs A/C is right next to the disapearring staircase. I've noticed that there seems to be a warm spot there during the summer, so it could be that the return is drawing attic air from the staircase. I'm thinking about putting some weatherstripping between the attic door, and the frame to seal off any air leak. If that doesn't work, I'm thinking about covering the gable vent with a piece of heavy plastic, and then taking it off in the summer. What do you guys think, and how do you think I should correct it? Putting in a gas fireplace is not an option. Thanks!
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