Coupe Quattro 20V ISV question

[schulz@iatcmail.ed.ray.com (Peter Schulz)]

Folks:

My 1990 Coupe Quattro is going through a habit of stalling and
irregular idling.  While driving, and then for instance, taking
the car out of gear to stop, the rpms will drop to 500 or below, 
causing the motor to buck, sometimes stall, sometimes return to 900.

I replaced the ISV two years ago, and I recently replaced the old
rubber crankcase breather with the upgraded metal replacement. 
Cap, rotor, plugs, plug wires were replaced last year.

I always run the CQ on Texaco 93 octane. As a test, I disconnected
the ISV lead, and the engine promptly died.  The engine would not
start with the ISV lead disconnected.

I removed the ISV, and cleaned it with carburetor cleaner, and then
thoroughly with brake cleaner with that ran out. After the ISV dried,
I reconnected it, along with the lead.

The engine idled better for a while, then returned to its old habits.

This problem seems to be accentuated by hot, humid weather.

Any ideas?  

Thanks!

Peter Schulz
1990 Coupe Quattro
schulz@ed.ray.com