I'm thinking in order for the valvetronic to cause a miss on 6 and 8 it has to be not opening valves on those 2 cylinders the same amount so the problem is unlikely to be with the motor or anything in common with all cylinders. That being said, I would make sure of spark plugs, coils compression, injectors, vacuum leaks etc. before blaming the valvetronic/valve lift. If it does come to that you could try to adjust the valvetronic manually or maybe adjust the minimum air flow at the throttle body or IAC if it has one.
I've never worked on these though, just my 2 cents on how I would approach it.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)