hunting idle on 5ktq

[blarson at Zeiss.com]

I have a problem, the idle speed rapidly varies from 1200 - 1400 rpm and
repeats about 1 time per second on my 1987 5ktq.  I have checked and
eliminated all of these items, and the hunting idle continues.

No codes
replaced idle switch(and checked with dmm), adjustment is fine, and wiring
checks good
idle speed valve changed, and if unplugged, hunting idle does not change
no vacuum leaks, and car dies if oil filler cap removed.
climate control turned off(ac not affecting idle speed)

The car drives great once gas is pressed, but when idle switch closes the
problem persists.  Anyone have ideas about this?  The thing that baffles me
most is that unhooking the ISV does not settle the idle down, which
eliminates the control relay and wiring in my thoughts.
Thanks,
Brian