you maybe referring to the fuel pump relay which is located on the firewall on the 560SEL. I believe it is the same for the 420SEL.
Any luck on your engine stalling problem? I have exactly the same problem in addition to a rough idle.
I will try and open-up the fuel pump relay this weekend for starters and see whether i have a solder problem.
I will also open up the injector lines to see if I have clogged-up problems
Do you have any tips in opening up the sealed relay casing? I am afraid I might mess it up. I sure do not want to damage the casing. What did you sue to reseal it after repair?