Unfortunately, that is somewhat the nature of the beast - the '79 has that wax pellet idle air control that is operated by the engine's water temperature, and it takes a few minutes for the water to get hot enough to operate the pellet. Make sure your engine thermostat is in good shape, and that the engine temperature gage shows the engine is warming up. I think the wax pellet starts operating the idle control when the engine temperature reaches 140 degrees.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C