Problems with starting my 420 SE (W126)

Hi there,

Cold or warm, I often experience some difficulties starting the engine: at the first crank it's revs up and immediately drops dead.
When I try again it won't start anymore. After insisting serveral minutes, it starts running again, quite normally.

For your information:
-Battery is in good condition
-enough fuel is in the reservoir
-fuel smell while engine's running
-irregular idling speed

What could this be: a mechanical or electronical problem...??

Do you have any ideas for me, please.

Thanks in advance,
Rémy K.