Sounds like the throttle body and/or Idle speed control unit are gunked up, the gasket connecting it to the maifold maybe perished or the breather hose(s) may be split/perished. Try the following and see how you go. If you have a Haynes manual you're laughing. The following might differ slightly as your model is not only auto but a different year to mine. Just a tip but I usually take photos as I am going along so I can put it all back together the right way.
1. Take the air box off by using a 5mm(?) allen key. Sometimes the heads
are chewed up on these bolts and you need to cut a groove in them to
get a big screwdriver in instead. That can be a pain. You may as well
take the whole inlet trunk off as one. There is a pipe on the underside
of the right hand side of the air box that leads to the Idle Control
Valve. Take this pipe off too by undoing the jubilee clip. Old type
original GM jubilee clips are 1/4" whereas the new ones if they have
been replaced are 7mm.
2. Reach your arm around the side of the engine following where the pipe
you just disconnected goes to. You should feel the valve and be able to
see it if you look hard enough. You need to unplug the ECU connector
from it by squeezing the edges.
3. There is a jubilee clip that holds it onto the inlet manifold, you
need to undo this. This is the trickiest part - If the person who last
cleaned it did it from above they will have left the jubilee clip facing
your way so you can get your 1/4" or 7mm socket with extension onto it
and undo, or a screwdriver if you find it easier.
4. Once undone pull it out from underneath the manifold and spray the
inside of it with injection intake cleaner. I use the Wynns stuff, about
a fiver from Halfords. Spray copious amounts in and move the valve with
a screwdriver or similar. The cleaner fluid that comes out should be a
bit dirty.
5. Give it a spray of WD40 to prevent sticking. Not really needed but just in case.
6. Place it back under the manifold remembering where it was located
exactly. The small black rubber pipe needs to be pushed back on (check it is still nice and flexible with no splits/holes). Easier said than done. Then another fiddly bit - Getting the jubilee
clip tightened from a remote location.
Then just reconnect everything as before.
Let me know how you get on.