Hi,
Agree with SMS almost certainly the torque convertor, had this exact issue with my C43 5 Speed.
I would start it and leave it to idle for a minute or two and it wouldn't stall out though the revs would dip very low.
I had the GB Oil changed and flushed and week or two after it no longer did it, though this was only temporary as it returned after about six months.
I had the GB rebuilt and TC replaced when i fitted a 55 AMG engine and of course no longer there.
I always believed it to have been caused by the TC.