You need a modulator adjustment. If you have a lift available. Remove the rubber cap and pull the steel key out of the castelated top. Turn it in (clock-wise) one full turn and try it for a few days. Your shifts should be noticable firmer. One turn is a minor adjustment but if your problem is intermittant you are just on the edge and this adjustment should be enough.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician