I have seen problems with ALL of the overvoltage relays. I would start there.
Also, I have replaced a handfull of the fuel pump relays. It is located in the trunk under the matt to the right of the spare. It is a green relay with a fuse in the top. I might throw one of those at it if I was guessing.
We sell 'em...
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS