'87 5KTQ stalling during deacceleration

[mark]

Hello Listers.

I have developed a nasty stalling situation recently on my '87 5KTQ.  It
only happens during deacceleration from highway speeds to a full stop.
I down shift while approaching the intersection with no problems.  The
problem usually happens when I shift from 2nd to 1st gear and am rolling
up to the intersection to Stop.  I noticed the tach in the car as this
occurs and the needle rapidly dips from around 1200 RPM to around 600
RPM.  When this happens the engine is on the verge of stalling.  From
watching the tach it would appear the car is overshooting the idle
setting and then struggles to right itself back up to 800-1000RPM.
Sometimes when this happens the car will stall.

I have searched the archives and came up with three possible places to
start looking.  They are: a bad overrun cut-out valve, turbo bypass
valve, and/or the idle stabilizer valve.  Based on the list's experience
what would be the concensus opinion where to start to look?

Since this problem would appear to be related to the idle I am thinking
it probably is the idler stablizer valve. Is my logic flawed?   Any
information regarding a potential solution to this problem would be
greatly appreciated!  Thanks. Mark