1990 Grand Am, performance problem -- exhaust, MAS?

Hello everyone,
I have a 1990 Grand Am (tech 4 2.5L), it runs, but doesn't accellerate right
any more, and the gas milage is getting bad.  It seems like an exhaust
problem, and I'm suspecting the catalytic converter, or the MAS (Mass
Airflow Sensor).  I tried taking the catalytic converter off to see how it
would run without it (isolate the problem before I shell out $70), but the
bolts are rusted on.  The exhaust pipe does change the airflow while the car
is just sitting there, and there's black stuff in the tailpipe.  My car
doesn't burn oil, just had the sparkplugs, airfilter changed, and the oil
was changed last month too, but it's black again.

The performance actually goes up and down, sometimes while I'm driving, it
will run well, sometimes it's in the gutter.  I've also noticed a little bit
of clicking, when I rev the engine in neutral, and it almost stalls if I rev
it up in neutral, and then take my foot off.

I'd really like some ideas from people, even not particularly relating to
this car.

Thanks,
Carl Busjahn
busjahn(nospam)@bellsouth.net