funny, just last week, a friend of mine, who owns a 1987 560sel had the exact same problem. I am his mechanic, and ofcourse it took a little for me to see this problem happen. indeed, rpm showed on the tach, but car simply died.
would start right up, sometimes would come alive before a full stop.
I suspected a bad fuel pump, replaced it, and voila, for the last week no more stalling, and actually the car now will reach redline, something it did not want to do before.
I recommend you either do a fuel pressure check. I did not do this, pump was very very noisy in my case, so was an automatic suspect.
Be aware much like this 560 sel I worked on, the 420 has 2 fuel pumps in the back, in parallel. It is unlikely they both fail at the same time.
So check with a stetoscope, or something similar to make shure that they both work.
In case one is dead, you indeed might get the symptoms you have.
In my 560 case, indeed one pump was dead. the other on the way to its final grave. I burried them both.
If both pumps actually run, than you have a power issue, or relay problem.

However my experience has told me that relays usually do not faill this way.
They will prevent a start, or an intermittent no start, but not a sudden powerloss. in the midst of operation.

May this help.

Henri.