2005 Chevy Impala Computer problems

My car died yesterday at work. After about 2-3 hours later I came to start my car and it worked. So I drove home and it died again on my way home. Anyway, it starts but it dies after 2-3 seconds. I parked it overnight at home and woke up this morning and started the car and it came back on. WTH.I took it to a mechanic and they said it was the ignition module. They put the new parts in and test drove it and it did the same thing. Now theyre saying its the car computer. Can anyone help me on this? How can you tell if the computer needs to be replace? What are the symptoms of a malfuntion computer?