Yes, the idle is too low and this is most likely the problem as long as the engine is in tune and the engine is healthy.
Sedans of the era had a hydraulic piston which kicked the idle up a bit when the car was at idle with the steering activated. The piston ,which was on the intake manifold, acted on the engine linkage and had a metal line which came from the PS box. I don't think any of the SL engines had this. But it looks like steering at idl,e is a period of very high demand on the engine. Too low an idle will cause the engine to stall.