Is yours a W113 or R107 SL?
As for the W113, I would look at the fuel delivery, linkages and settings (I had the symptoms you describe once, and it turned out to be a loose linkages bracket on the intake manifold), spark plugs and wires, ignition timing/dwell - in short, the basic tuning stuff.
Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic