Possibly a vent problem, but I would still opt to have your main line water jetted upstream (towards the house) then out bound.
You stated you have a laundry room and soap causes a scum, wax build up on the internal piping walls which could restrict the flow like a partial stoppage.
The water jetting, if performed properly would scour the piping to a like new condition DO NOT USE USELESS Chemicals.
Good luck.