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ISV valve

I drive a 1990 Audi Coupe Quattro 20V with 200k km, and I have been having 
some idle problems lately which I am almost certain are due to the ISV 
valve. The initial fault was a no idle what so ever condition with no other 
performance changes above idle, this occurred immediately after about 150 
km (90 m) of 150-180 km/h (90 m/h- 112 m/h), driving on an open highway 
with the oil temp approaching but not exceeding 130 C (266 F). After 
reading some background engine problems advise on the 20V coupe quattro 
page I removed and cleaned the ISV and it worked fine, for about 20 K of 
city driving. The idle problem then returned quickly but not all of a 
Next day I soaked the ISV in degreaser for about 4 hours and the car again 
worked fine for about 50 K of city driving, but performance is now 
beginning to show tell tale signs of fading, ie it has to hunt a bit for 
900 rpms when I let of the gas pedal and it occasionally stalls.
I am going to give the valve one more soak followed by a good blowing with 
an air hose but my question is this, is this valve on it's last legs or 
might there be something blowing in from the crankcase breather that's 
plugging it up, or might there be another source for this fault?

Thanks kindly