Hmm..Been struggling with this same problem: car stutters right before switching over to closed loop. Plugs ain't the problem, I can tell you that. Its something in the fuel system. Coolant sensor?

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'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E