Manual Shift Volvo 850 randomly goes to low idle & stalls when coming to a stop.

1994 Volvo 850 wagon - std fuel injected engine - manual shift. Normal idle is 850 RPM. Usually very steady. When coming to a stop, I usually depress clutch and downshift, maintaing clutch depressed. Randomly, idle keeps dropping past normal 850, sometimes recovers, sometimes goes to zero (i.e. car stalls). No problem restarting. Any ideas? Is there an idle stabilizer/injector of sorts that could be faulty/intermittent? Have used 3 cans of injector cleaner - no change. Thanks for any advice.