You've heard this one before - drains are slow, but I've got a twist. All the drains are slow: tub, sink, toilet... I can plung and they are fine for a week or so then slow again. I don't think it's roots, plunging shouldn't make a difference if that's the case. I used chemicals, same results. I've checked out other Q's and A's and have read about clogged vents as a possible cause. How would I go about checking that? Hear's a little history - house is 50 years old. Hooked up to municipal sewer. Addition built 5 years ago with two bathrooms. Addition has own soil stack directly to original sewer outlet at basement wall. It also has own vent. The problem affects both old and new fixtures but at different times (they alternate). Are you intrigued yet? The only plumbing fixture not affected is the kitchen sink- it's closest to the drain, farthest from the soil stack, but it's not vented!! Yeah I know that's not legal, but it's not an easy fix either. It's original and on an exterior wall with a window over it, so I can't get to it. Other then the occasional smell and gurgeling it works great. Any ideas?

P.S. What I've read so far is impressive. Please help, thanks.

All the drains are slow: tub, sink, toilet... I can plung and they are fine for a week or so then slow again.
What may be happening is that grease and hair may be plugging the drain and you opened a small opening when you plunged that closed up in time. Try snaking the lines and flushing them out with boiling water. Good luck, Tom