When you video the lines do it from the bathrooms, dishwasher, and washer. Also with everything running at once, you may have a problem with the vent being blocked or partially blocked or inadequate. Some plumbers start rodding with the vent on the roof. Leaves, twigs, dead birds, dead squirrels, etc. can plug off the vents causing the same problems as well as being undersized for the amount of sewage being moved. A plugged vent will not drain right and will act like pouring something out of an upside down coke bottle(glug, glug) and a slow running drain will then plug up. Remember also the washer will drain some lint and soap bubbles create air pockets in the drain and the dishwasher will dump food chunks in the drain. Plumbers will also run the garden hose down each vent on the roof to see if it is plugged and to clean them out. Besides feminine rags, I've heard plumbers complain about condoms and wads of Charmin which doesn't dissolve as well as other brands. Some people put it in their rental contracts to not flush things like rags, condoms, etc. These things commonly show up in the screens at city sewer/wastewater treatment plants.