The intermittantcy of your problem suggests a bad solder contact in the idle speed controller. This is separate from the engine management controller on those V8s. Its somewhere on the passenger side under the dash. Maybe under the floor panel. SLs always give me grief on component locations.
This is a very solderable problem. We get very good results resoldering the pc board for symptoms like yours.
Bosch Master, ASE Master, L1
33 years MB technician