[s-cars] Help - car is not holding idle while at stopsigns, stoplights

[marc weiner]

etc.  The idle starts droping below normal and then the car eventually stalls - just started happening today in our zero degree weather here in Ft. Collins this evening.  I can keep it from stalling by keeping the revs up while stopped but it is a bit of a pain.   Is there a quick inexpensive fix?  Is it the ISV?  any advice would greatly appreciated.  Had the alternator replaced about 6 months ago and the battery is fine.  Car has 230,000 miles on it.  
94 S4