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S4 cold run problem

Hello Everyone,

I've got a strange problem with my '92 S4, and I'm looking for any
suggestions or BTDTs.

'92 S4, purchased last June.  The original owner had said he replaced the
timing belt.  However, the Auxiliary belt broke in Sept/Oct, so that was not
true.  The timing/auxiliary belts and water pump were replaced by the local
Audi dealer, and they say there was no valve damage.  

As the weather got colder, the car would kick or stutter when cold (5 hrs+
cooling) and the ambient temp is below 45 degrees F.  The problem gets worse
as the resting temp gets lower.  The problem seems to be amplified if the AC
compressor is running.  It feels as though the fuel flow to the engine is
cut off altogether, not like a sputtering: It is more violent a kick.  The
problem continues until the car has been driven for 15-20 minutes.

I have checked/replaced the following:

Checked/tightened most hoses and their clamps
Swapped ECUs with another '92 S4
Replaced Engine Coolant Temp(ECT) sensor
New Bosch Plugs
New valve cover gasket (needed independent of this problem)
Check error codes by both jumping the connectors as per Scott Mockery's
page, and through the Climate control system.

The local dealer replaced the engine RPM sensor (VAG code without the engine
started??), and then sent me to swap the ECU because that was their next
best guess.

A local Euro/Audi shop has tested or replaced the following:
Coil pack was swapped with another S4
Another ECU swapped out
Airmass sensor swapped
Many grounds were checked
The ECU Pinouts were checked
Checked O2 sensor
Injector Coolant switch (I think that was the name) was replaced.

My current thoughts are maybe the wastegate frequency valve, as it can
trigger an engine overspeed condition (I think):cost ~$70

The local shop suggests that it may be a ground they did not test yet, or
the throttle position (TPS) sensor. Cost ???

Any ideas??  I don't know what to do next, and the car runs horribly.
Thanks in Advance.

-Scott Thompson

92 S4 (badly ailing)