Idle speed oscillates. Not ISV, I think.

[Smeins, Larry]

My 87 5kcst has developed the dreaded idle speed fluctuation.  This one is
different than my previous experiences.  It rapidly oscillates between 1300
and 1500 RPM at about a two cycle per second rate. Each time the speed
begins to increase the engine stumbles a bit.  Since the first thing
suggested for idle speed anomalies is the ISV that is the first place I
went.  I disconnected the electrical connector from the ISV and no change.
Even so I removed the ISV and cleaned it, again no change.  I then went
looking for vacuum leaks.  When I remove the dip stick the oscillation
frequency decreases a bit.  When I take a propane torch, spewing propane,
around all available vacuum lines and ports I get no change except for when
the propane is sprayed into the air filter snout, which increases the rate.
This problem has been going on for about 5 days and twice the idle has stuck
in a high state, 2200 RPM, but I have not been able to purposely induce this
condition.  Another thing that is happening is when I am running at speed
and remove my foot from the accelerator there is a delay of 1 to 2 seconds
before deceleration starts and then it is very abrupt.  It is not a smooth
transition like before the idle anomaly started.

All help appreciated.

Larry from Loveland, CO