intermitten trouble starting
 

Hello,
I have a 1999 E320. In the past month my car on two occassions had trouble starting. The first time my wife was able to start after 10 minutes of trying. I took to shop and they had it for five days and found no codes and didnt have problems starting. 3 weeks later she gets stuck at supermarket-calls AAA and they come out and get it started by using starting fluid. Is it Fuel Pump?filter? any suggestions