It sounds like the combustion blowers are pressurizing the firebox and pushing flue gasses out. Time to get a draft gauge of some kind, that way you won't be guessing.
Look at the inlet of the combustion blower, there should be something to adjust how much of the opening is exposed. Try closing the shutter 50% and see how it runs. Be very careful, you do NOT want to succumb to CO fumes!
crazy4coal wrote:I would take the 90s and turn them so they are 45s to straighten the pipe out. Make an offset instead of a sharp turn.
Good idea. One more thought...is there plenty of fresh air available to the room that the stove is in?