Do a search on Fuel Pump Relay. You can jumper the terminals when the car won't start in order to eliminate it as the culprit.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280