Idle drop on 4kq

[Roa, Greg]

Hello All,
I've got a question.  I adjusted the idle on my 4kq a week or two ago, by cranking down the screw on the side of the throttle body.  Idle is great now, and the car runs very smooth.
Problem that I have, is that on occasion the idle will drop down to 300-500 rpm when I let go of the throttle, and push in the clutch.  Most of the time it is fine, but every once in a while this will happen.  It gets low enough that the instrument cluster lights up.
Anyone have any idea what might cause this?
Car is a 86' 4kcsq.

Thanks!
Greg Roa
Cincinnati, OH
86' 4kcsq
93' 90 CS
83' 944