If you take the intake tube off the plenum (the thing marked ECCS), you'll see a flap that is moved by the throttle cable. Just before that flap, there is a small hole which is involved in idle air control. Head down to Autobarn and get yourself some carby/throttle-body cleaner. Disconnect the tubes that run from this hole and clean them out by spraying in some throttle-body cleaner. Basically just clean that air route. While you're at it, clean the throttle-body and plenum chamber as well. Put it all back, start it up, run it awhile, give it a couple of revs... then let me know if that's helped.