Probably the turbo bypass valve leaking.
http://www.saabcentral.com/techhelp/c900/idling_problems.php .Fred's idling guide said:On a Turbo the Dump or Bypass Valve (Classic symptom "When I come to a quick stop, usually in traffic or when someone cuts me off or at a red light, the idle drops from just under 1000 to almost zero and the car stalls.") The diaphragm in these plastic valves fails and leaks vacuum. If you can suck air through the small-bore pipe to the valve (which is down front of engine above turbo) it's broken; they are reasonably cheap (£20) and quick to replace. Sometimes failed hose clamps on these valves cause the same stalling problem.