This is a difficult problem for which there seems to no set answer. I would check the roof vents for obstructions and proper location. The odor could come on days with there is a temperature inversion. This would be similar to a fireplace chimney backing up during temperature inversions. This generally occurs when the temperature turns cold and cold air forces the warmer air down the chimney into the firebox and odors result. I would also check for obstructions in the plumbing system that block the flow of air to the roof vents. Could the baffles on the septic tank be damaged and thus preventing the pressure equalization. There could also be a crack in one of the pipes or lines which could release gas under the right conditions. When you finally solve the problem, I would be interested in you calling and letting me know what it was. Yours is not the first inquiry I have had about this, but I do not know what the solutions were.