I have a 1990 Olds Cutless Supreme, I have problem with RPM drop.
Sometimes when stopped at light RPM drop low and when I go to take off car stalls.
Usually if the air-conditioner is on the RPMS stay up and engine runs fine.
IAC valve sluggish , carboned up
Possibly dirty iac. But if running fine with ac on I tend to suspect dirty throttle plate causing your issues.
It was the dirty throttle plate. Sprayed Carb & throttle cleaner and watched the dirt run out. Did this 5 or 6 times and took for a 20 mile test run and it runs like a charm. Thanks