You will find the Factory Service Manual at Mitsubishilinks
The page is setup so you can donate to the host of the FSMs and help with his cost to keep them available.

The IAC is next to the throttle position sensor closer to the firewall.

Is your check engine light on? Do you get the erratic idle at startup or only after the engine warms up? Have you checked your coolant level?

1990 LWB RS A/T
1990 LWB Base A/T
1991 LWB LS A/T Going...Going...