Confused new lister

["Jeffrey Berlin" <JTBerlin@email.msn.com>]

Hi,
I am confused. Over the past few days, I have gotten some good info from
some of you-- thanks. But I am now not sure what the O2 sensor really does
and whether I need a new one. My car has hesitated some, only recently
running rough and idle fluctuating between 900 and 1200. I have a 90 CQ,
later serial #, and I have a chronic Idle problem since 91 when I bought the
car new from the dealer w/24 mi. on clock. Dealer could never fix it. I also
was told I need a hot-wire mass air flow sensor. What is that? And what does
it do? My car won't idle at high altitudes (9,000' and up). And lastly, does
my car have an altitude sensor? If it does, would that be the key to my high
alt. problems?
Thanks,
Jeff