The 89' has a "kicker" on the throttle body
that when the engine is cold kicks the throttle
open a little to give you a high idle.
If this is kicking on and off, it could affect the
idle speed. I have always had too high idle when
cold and erratic idle at times, I think it's the way
the throttle body was designed, I think there is a
screw to addjust the idle speed that the kicker gives,
try backing this off some, the other screw adjusts
the regular warm idle speed, it does not contact
the kicker, so you should be able to tell which one
you need to addjust.
I think the kicker is a piece of junk, it's vacuum opperated
and if you want you can adjust the thing so it doesn't do hardly
anything, and just run from the regular idle mostly.
there is two little electric opperated valves at the front of
the engine on the intake side, they got hoses running to them,
those might be the problem too, mine got both vent covers
broken off, (bonehead dealer mechanics) but I don't know
what they do, or if they are needed, seems to me one of
them is the vacuum line that controls the kicker too.
Check the vacuum lines for proper conection and placement,
I know all of mine probably need replaced, but I don't have
any idle problems I can't live with.