Alright I've dealt with a recurring bad smell for about 2 years now,
and after multiple visits from the plumber (unfortunately the smell was
never around when they came) and multiple posts on usenet (the answers
to which have tended to be a lot more informative than what the plumber
told me), I think I've narrowed down the problem to the p-trap that's
connected to my central AC drainage pipe.

The smell used to come when I ran the water in the kitchen. I got the
p-trap cleaned out (it was VERY clogged), and the plumber added a
t-joint kind of thing with a cap so I can occasionally pour bleach in
to keep it clean. Strangely, the smell doesn't come when I run the
kitchen water anymore, but now it just comes around randomly, usually
near the end of the day. Not a pleasant thing to come home to after
work.

I -think-, but I'm not sure, that the p-trap is either vacating due to
some weird pressure thing in the plumbing, or just not filling as fast
as it evaporates. Could someone more knowledgeable than me tell me if
either of these make any sense? I've occasionally been putting water
into that cap thing he installed, but it hasn't made much of a
difference.

When I do a search for "p-trap" on google images all the ones that show
up have a much steeper angle than the one that I have installed. I had
foolishly not requested that the plumber put a new one in (I asked him
whether he would replace it, he said he'd just clean it out which would
save me money). Is this a possible problem?