Henri,
You've got a good one there. I read your post carefully, and it looks like you've covered everything. Now it's straw grabbing time. I can only come up with two longshots. Possibly there is a restriction in the intake runner. Also I've seen lifters on newer MBs cause this problem but never seen it on older models like the 560s. You could swap lifters with another cylinder. Let us know what you find out. This is a very interesting problem.

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