'82 urquattro idle problem

[ejfluhr@austin.ibm.com (Fluhr)]

My '82 urquattro does not want to idle smoothly.  It tends to fluctuate
~200 rpms up and down, from around 800 rpms to about 1000 rpms.  Also, it
intermittently stalls when idling or when returning to idle from higher in 
the rpm range; this occurs regardless of whether the engine is hot or
cold, although I think it might be slightly more prevalent when the engine
is cold/warm.

I have checked the idle and WOT switches, and they are both working fine
(less than 1 Ohm resistance).  I have adjusted the frequency valve duty
cycle such that it fluctuates around 50% at idle, and the oxygen sensor
appears to operate normally around 0.5v.  The duty cycle also reads
around 40-55% when driving normally under vacuum or non-WOT boost pressure.
When the car is at speed and under WOT, I get a duty cycle of 75.x%.
If I trip the WOT switch at idle, the duty cycle reads 48.8%.

I don't have any other driveability problems that I have noticed (aside
from what I believe is an occasional overboost cutout), and the engine
pulls ~16InHg vacuum at idle (VD0 boost guage...although it might be off
by 1-2InHg).  I have two good engine-to-chassis ground straps in place,
one of which is at the intake manifold ground point for the ECU.

Just once I happened to catch the frequency valve duty cycle when the
engine stalled after returning to idle.  It appeared to read much lower
than normal (around 30%, I believe).  I am not sure why this might have
happened, unless the ECU drops the duty cycle when in deceleration
conditions (i.e. throttle closed but rpms > idle threshhold).

Any ideas, aside from "Welcome to urq ownership!"?

Thanks,
Eric
'85 CGT, '82 urq
---
Eric J. Fluhr                                Email:  ejfluhr@austin.ibm.com
630FP Logic/Circuit Design                   Phone:  (512) 838-7589
IBM Server Group                             Austin, TX