'88 90 weird idle

[Andrei Kogan <abk@phy.duke.edu>]

Hi,
Perhaps this is to contribute some more to the ISV thread. The cars starts
perfectly (warm or cold) and idles at 900-1000 rpm. After it warms up (and
I think it is not just the temperature, but rather driving for a little
while at a highway speed) the idle drops to 400-500 ( according to the
in-dash tach) and quits occasionally. Do I start with the ISV? Perhaps
unrelated, but here are a few more curious symptoms.
First, when accelerating, I notice sporadic changes in the engine power,
esp in 2nd and 3d. Not VERY noticeable, but it seems to hesitate a little
when it revs upt to 3-4000. 

Second, Could someone please tell me   what the signs of a failing cat are?
My exaust rattles quite bad (especially on that low idle --- it is *awful*)
and I took the car to the muffler shop were we spotted a couple of busted
hangers, so I thought it was just bumping against the car bottom. The
rattling is still there, although it seems to have gotten weaker. I heard
that rattling in the exaust often indicates a bad cat, but could someone
please elaborate. What are the other signs?  


Thanks alot.
Andrei

Oh and congratulate me, my wife can get into the 4000S through the driver's
door again. It took us almost three years and 2 hours. The door handle
turned out to be pretty easy to replace, although there appears to be a
unique sequence for attaching those three linkages.