Intermittant power loss on A140

[Birdman]

The M-B garage has no answer for it because it can't reproduce the problem or identify it from the system's diagnostics. The engine occasionally loses power as revs rise, so far mostly at town speeds, but on changing up normal power will return. The car has a pre-selector, semi-auto box. It remains an intermittant problem but has resulted in the car coasting along without power so is potentially hazardous on overtaking. The vehicle is 18 months old with 6K miles and has been serviced by M-B. Anyone know anything about this type of problem?