Thanks so much for the replies, it's great to hear from experts and those experienced in the field. Responses to your feedback:
-we had a camera inspecting into the main sewer line in December, which is when they flushed the sewer line. I'm the end unit, with the sewer line running away from...
I've had a problem for some time, and i'll recap the best I can:
-owner in an end-unit of a string of 4 condos (built in 2006)
-plumbing system includes usage of AAVs
-2 stacks, and a shared vent that releases above one of the other condos
-condo is 2 stories, and includes a basement
Issues...