With all the suggestions I haven't seen to many that deal with what the problem is. I have seen a few that are part of the problem and these need to be addressed. This is vent or exhaust. Your outside pressure is overpowering your firebox. So look at obstructions for sure. Your problem is the design of this stove and the horizontal outlet or vent. For this stove to work well, you should cut a new exhaust in the top of the stove, or at least remove the elbows from the 2 I see (which means there are 3). This stove wont vent properly until you have a single elbow immediately after the rear vent, then go straight up.. Your problem is the stove is sensitive to this even with the single rear vent. Now that you have added elbows that sensitive stove cannot compete with outside pressure because of it.
