Volvo: 850 Volvo stalls when stopping, volvo 850 wagon, vacuum leak

Question
1994 Volvo 850 wagon - std fuel injected engine - manual shift. Normal idle is about 850 RPM. Usually pretty steady - hot or cold engine. When coming to a stop, I usually depress clutch and downshift, keeping clutch depressed. BUT LATELY, randomly, idle keeps dropping when i come to a stop, past normal 850 RPM, sometimes recovers, sometimes goes to zero (i.e. car stalls). Never a problem restarting (and if I let clutch out before stall, engine RPM recovers) and all is fine again. Power normal, shifts normal, etc. Any ideas? Is there an idle stabilizer/injector of sorts that could be faulty/intermittent? Could it be a dirty throttle body? I have used 3 cans of injector cleaner in gasoline - no change. Thanks for any advice.

(And, oh by the way, car engine whistles when cold in winter (say, below 40 degrees F) until it warms up. The kind of sound I guess that air would make passing over a small opening like a flute... but higher pitch... varies with speed of car (faster = higher pitch)). Does this suggest a problem with air intake? Has been doing this for 12 years!!! Dealer stumped and we've given up on that one.  

Answer
Allls,
Look at the hose to the air filter and make sure there are no cracks in the hose. Also look at the throttle housing making sure that it is clean. Use carb cleaner spray to clean it out. The Idle speed motor is just in front of the throttle housing, make sure that it is plugged in. Check for codes that also may leed you to the problem. The whistling sound suggests that there maybe a vacuum leak. Check all the vacuum lines and along the intake manifold for leaks. Some models have a vacuum fitting just behind the power steering pump, in the front of the manifold.