If you can get up on the roof you might want to try a temporary extension to the chimney to see how this helps. Get a 4ft length of 6" round warm air duct pipe from the store. On a calm day, pull the chimney cap and cram the vent pipe, crimp down into chimney. If you can, anchor it with a couple screws. Fire up the stove and see how she runs. If it's radically better, the issue is marginal draft.