Very common M103 problem..
Here are things I would check: (from cheap to expensive)
- fuel pump check valve (~$15 at partsshop)
- fuel accumulator (next to fuel pump, ~$120)
- fuel pressure regulator (under air cleaner next to fuel distributor, >$200)
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".