Another possibility -- and I speak from recent experience -- is your electrical system. If your alternator isn't charging your battery properly, or if your battery has trouble holding a charge, your engine could stall intermittently.
This happened to me when I installed my new sound system with three new amplifiers. All it took was a half an hour of listening to the radio without the motor running to partially drain the battery. Although the battery still had enough juice to start the motor, the stock alternator with its 55 amps couldn't produce enough juice to charge the battery while the stero system was on, and the car stopped dead in its tracks until the battery could recuperate or be jumped.
Upgrading to specially modified 100 amp alternator and fully recharging the battery solved my stalling problem.
1985 190E 2.3 8v
Rebuilt engine May 2002 (at 137,000 miles)
Lowered with Eibach springs and Bilstein performance shocks
An embarassing number of new parts
Pioneer DEH-200 head
Infinity Kapa Perfect 6.5" speakers in front doors
2 Sony Explod 8" subs in sealed trunk box vented through first aid kit
3 MTX amps, Callisto electronic crossover