I read about this exact problem somewhere in here about two years ago, unfortunately I've forgotten what the outcome was.
What happens with my 1991 300SL is this; When I first start the car (in cold temp) it starts perfectly. When I start the car after it warms up or in warmer temps such as summer the car starts then shuts off. I turn the key back to 0 and start it again then it starts and stays running!!!! Sometimes it will run very rough for one to five seconds then smooth out.
When I put it in 'drive' it idles at 500-600RPMs and there is a rattling sound coming from some unknown location. When I shift it to neutral it revs up but only to 600. With the AC on it kicks the rpms up but only for a moment. If this is a sensor please tell me where it's located. If I remember correctly I read that this is a high idle sensor but I think it would be a normal operating temp idle sensor!?!

Thanks in advance