Susan, Vehicle should be checked for trouble codes to diagnose the problem, could have a faulty fuel injector or a vacuum leak. While car is running tap sharply on each injector about 3 times, sometimes this will un stick a injector temporally and engine will run smooth.
To locate vacuum leaks I use engine starter fluid in aerosol can and squirt around vacuum lines, intake,injectors,etc. till I locate suspect leak, you will hear a difference in how engine runs when starter fluid is sucked by vacuum leak.
Check the attached links,instruction and guides, Good luck
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How to Fix Rough Engine Idle Problems at YouFixCars com
Rough Running Engine James on Engines 2
The Lost Art of Diagnosing Engine Problems

Hi Susan, The link below is a short youtube video showing how to test and fix a car that shuts off randomly, Hope this helps.....CHEERS..https://www.youtube.com/watch?v=oBo-vmEEwv4

CHECK YOUR SENSORS,