E320cdi 2003 Erratic Revs

[roonsta]

My E320 cdi seems to have a problem with the revs/transmission under normal conditions. I can let it tick over 1st thing in park with no problems. After driving for around a minuite or two the engine revs start to waver in Drive between 1000 - 2000rpm. I have found that dropping into 4th or 3rd helps till the engine gets to normal operating area, although still not 100% stable. I have taken him to a merc specialist and had a star diagnostic test but nothing untoward is apparant - although at the time the engine had been driven for about 45mins so was up to temp. When driving on the motorway @ around 80mph the revs are steady @ around 2300rpm. Any wise men with any ideas? I would be really greatful.