RE: Toilet/Shower drain help.

["Gerald J. Brazil" <gerrybraz@voyager.net>]

I had just such symptoms and everything I tried failed. I finally resorted
to calling a plumber who repeated all of the steps I had taken with the same
negative results. He finally took a drain snake up on the roof and ran it
down the vent. Guess what he found?.....the remains of a dead bird.
Apparently a bird perched on the vent pipe and was gassed. He was small
enough to fall in the vent but big enough to plug it, making things not work
right.

An oddball problem, but I'd suggest snaking your vent.