Fireplace smell upstairs

When I have a fire in my fireplace, I notice a strong smoke smell upstairs. The smell isn't there until the upstairs A/C comes on. I went in the attic last night, and the smell up there is awful. I think know what the problems are, but I don't know how to fix them. Problem 1: The top if the chimney is adjacent to a gable vent on the end of the house. It's an older house, so the chimney isn't as tall as it should be. That has to be how the smoke smell is getting into the attic. Problem 2: The attic door (disappearing staircase) is right next to the return vent for the upstairs A/C. I think that the door leaks air into the upstairs. I'm going to put some weatherstripping on that door to hopefully seal it, but I was also thinking about putting a piece of heavy plastic over the gable vent on that side of the house. Is that a bad idea? What's the best way to fix this?