It sound very much like a problem with your fuel relay. If your car is similar to my 300E regarding the fuel relay... then there is the strong possibility that the circuit board of the relay has hairline cracks where the componets of the board are soldered to it. Te components that normally cause a crack are the heavier miniature electromagnet and contact point assemblies. There are two or three on the board. Look on the back of the board where the pins from these littlle guys poke through the board and soldered. You may see circular cracks around the pin. If so buy new relay or resolder the spot (s).
Because of the weight of the electromagnet, a significant change in the angle of gravity can cause the heavier parts with cracks to shift on the board and by so doing lose contact with the circuit of the board... thus causing your symptoms... intermittant or no fuel pressure.
