If your car doesn't start after much cranking, check to hear if the fuel pump energizes when you first switch the key to the "on" position. On my car, it will buzz briefly (less than a second) before cutting out. If you don't hear the pump energize, it may be a shot fuel pump relay that intermittantly fails. I replaced it on my 300e, after successively more often no-starts were occuring, and thus far it has started everytime since (fingers crossed). If the fuel pump IS energizing, then it may be a vapor lock problem...
(and, my car does take two cranks to start most of the time..!)