I am having an issue with my plumbing backing up. It seems to happen about once every 2 to 3 months. I usually wind up having to go to Home Depot and renting a snake (at about $60 each rental) and snake it out. After snaking, it works just fine, but low and behold, a few months later it does it again. It seems to happen mostly when I am doing laundry, but does happen other times. Any idea what could be causing this. I am on city sewer system and have the access pipe at the back of my house.