You know alot more about MBs than I do, so I sort of feel silly responding.
Your situation does mimick that of a carb-engine with a plugged rear fuel filter. Have seen Acoords do this more than once. MB maint manual indicates 60K service for that beer-can fuel filter your 103 motor has. I personally think that's too long. I never let mine go more than 20-25K. How long has your's been on?
If it's been awhile, I'd back your 300E up on some ramps for easier access and remove/replace the filter for starters. OEM Mann/Bosch units are around $25. Dealers prices on these things more than doubled about a year ago.