R230 SL350 stalls / limps

Hi
Any suggestions on cause of SL350 stalling at or limping away from a stop once fully warm? Tends not to happen on shorter runs and doesn't always happen on longer runs either - not very helpful for diagnosis! Seems to be more frequent in hot weather. Can make pulling out of junctions or roundabouts interesting. If it doesn't stall, it limps along running very uneven and with no power. Gentle tickling of the throttle keeps it going, while a firmer push tends to finish it off or achieve nothing. Used to cause engine management light to come on, but it doesn't seem to do that now. Apart from these low-rev situations, it runs perfectly.
Diagnostics have indicated either nothing or a misfire. Garage suggested replacement plugs and leads but made no difference. They are suggesting another oil change thinking that might clear hydraulic tappets - valid or nonsense?
Any help would be fantastic,
Thanks,
Peter