While your suggestion is not beyond the realm of physical possibility,
any car that had it's idle speed largely dependent on the rate of fuel
flow from the tank would be really scary.

I would give that a try. Actually, *I* would take it to my mechanic
who has all the diagnostic stuff and have him fix it but since you're
shooting in the dark, give that a try. It certainly can't hurt and
self-repair is cheaper than paying for it. Self-diagnosis OTOH, can be
rather expensive in the long run.

While the O2 sensor may be weak (Bosch recommends 60,000 mile replacement
intervals.  But keep in mind they are the primary manufacturer of this
costly product), I don't think it is the source of your problem.  I agree
that a substantial O2 sensor problem would trigger the check engine light if
that was the primary cause.  Also, the O2 sensor affects fuel mixture (and
fuel economy) more than having to do with erratic idle speed.  From your
latest update/description, it is more likely that you have a
cracked/intermittently disconnected vacuum line somewhere that is playing
havoc with your idle speed.  This is something you can check yourself
because parts are very cheap (snip the end off a hose and reconnect to
verify the problem), but very labor intensive.  Professionals charge dearly
for their time and don't always do a thorough job when it is this boring.
Don't confuse/misconnect the hoses.  That is when the professionals charge
the really high labor rate to correct an amateur's mistake.