Also check the fuel pump relay. Actually, when it fails to start, check to see if the fuel pump is energizing for a second or so when you turn the ignition to "on." If it isn't, the relay isn't sending power to the fuel pump and it's faulty.

Sometimes the relay fails intermittently - my 300E had this problem a few years ago, and it would come and go. A new fuel pump relay and it hasn't done the same thing since.