A-class automatic transmission problem

I have a 2003 A170 CDI automatic that has a problem with the autotrans. Sometimes, when shifting to the next higher gear, the
transmission slips and the engine revs up. The car has 21,000 kms but has done this since new. Happens about 3 times per week, so it's never done so with the mechanic driving (so he says).
The Merc dealer refuses to work on it (it's under warranty) saying that the gearbos is perfectly OK.
Anyone else have an A-class automatic with this problem? Does anyone know if the autotrans mounted is a "real" one, or some sort of robotized gear shift with the infamous automatic clutch as mounted on some A-class model?
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