High idle problem on 87 5kcstq (2500-2800 rpm)

[blarson at zeiss.com]

I have been experiencing a very high idle on my 1987 5kcstq lately.  The idle
occasionally is normal, but usually is up around 2500-2800 rpm.  I have swapped
out the idle air bypass valve on the intake manifold with a known good one, and
the temperature senders on the coolant neck both measure the correct resistance,
and have no corrosion on them.  If the idle air bypass valve is unplugged, the
idle drops to normal.
Has anyone else had this problem with another component other than these two?  I
searched the archives, and these seemed like the most likely culprits.  Your
help is greatly appreciated.
Thanks,
Brian