why does car stall?

There a quite a few things that can cause your engine to stall intermittently. One question to as is - Does the engine stall when you let your foot off the gas or coming to a stop? If this is the case then the cause could be a dirty throttle body or sticking idle air control valve. If the engine stalls on acceleration then some possibilities are are a faulty crank shaft position sensor or mass air flow sensor. If the engine will not restart right away that might indicate a faulty ignition module. The powertrain control module is also a possibility in any of the above cases. There can be a lot of trial and error in trying to isolate the cause of an intermittent stalling condition. Any stored fault codes in the powertrain control module may help.