Mine does the exact same thing, its a problem with the Amp, or both the Amp and acutator. Personally I have given up on the stock system. I got an Audiovox vacuum controlled aftermarket system on ebay a week ago, and when I have some time (it has a LOT of pieces  ) I will get out there and put it in....it should prove more reliable than the prehistoric system in our cars.
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'15 GLK250 Bluetec 118k - mine - (OC-123,800)
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'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
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