I know things were addressed a bit with venting last time around, but I still think this may be where your issue lies. The vent(s) that extend through your roof can be blocked by animals, bugs, bird nests, etc. (which I'm guessing isn't the case). It can also be sloped incorrectly (should be sloped towards the drain/septic). An incorrectly sloped vent can fill up two ways - when its raining hard, and condensation. Weather this time of year is pretty prone to cause condensation... I'm betting there is a horizontal section of your vent line that is filling with water and causing most of your issues. You can try inspecting the line from the roof, but you may also have to get into the attic to see if there is a horizontal section.