was driving to work on friday and the car stalled on me on the freeway. wouldn't restart, waited about 20 minutes then the car restarted. went about another mile and then the car stalled again. this time it would not restart. went back the next day car would still not start. took to pep boys for the to do daignostic on it but to my surprise the car started right up just fine. they said they can't find a problem with it if it's running. any suggestions on what is causing this would be greatly appreciated.