A loss that significant sounds like fuel starvation. Change both fuel filters (the in-line filter and the spin-on cartridge) and report back.

EDIT: just saw that you already have a new fuel filter. Did you change both filters or just one? Make sure they're both clean.

Also, take an air compressor, take off the fuel return line to the tank, and blow some air backwards through that line to blow any grit or algae off the strainer in the bottom of the tank temporarily, just to experiment.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL