Sounds like you might have an out of adjustment throttle switch. During decel the actual degree that the engine brakes the car has a lot to do with wether it gets fuel or not. Above about 20-25mph the fuel is interupted by reverse current being applied to the EHA, whenever the throttle switch goes closed. If you had the ability to turn this off and on at will it would feel like the transmission shifting it is so distinct. During normal decl at 60 say, the first effect is slight and then the decel cut-off takes place and it is distintly noticable. It sounds to me that you are going in and out of this at subcruise throttle.

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