Thanks for your speedy response. The clog is definitely in the horizontal
run because I can take a snake and it doesn't get at the clog for several
feet. If I plunge, I can usually get it unclogged as well but you can hear
all the action in the drain pipe and I lose all water in the toilet (etc).


That last tidbit pretty much guarantees that it's a venting issue,
doesn't it?

If you've got a solid plug of matter moving down the pipe,
and no way for air to get in behind it, of course it's going
to stop moving.