Hi Eric; Have you considered the height of the vent pipe in relationship to the roof pitch? Many of the houses in this area have very steep roof surfaces and the wind follows the surface. I've found some vents that were two feet above the roof surface but still backdraft because they are catching wind from above. Some have blown the pilot lights out. Post a photo of the roof with the vent visible if you have one.

The only other things that come to mind are restrictions in or incorrect pitch of the vent pipe.