If the fuel pump relay isn't making good contact all the time, sometimes the fuel pump will run, and continue to run until you shut off, and sometimes it won't, in which case the car will either start and immediately die or not start.
MB relays are notorious for cracked solder connections as they age -- depending on the temp, phase of the moon, whether your rabbit's foot is sideways or not, etc. they will sometimes work and sometimes not.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!