Hi!
My '88 260E has the same symptoms but obviously nowhere near as bad.
Over the last 6 months or so it has become sluggish at low engine revs. At higher engine revs (2500-3000 and over) it goes great.
I've done fuel filter, air filter, plugs, checked valve timing when I did timing chain.
I was wondering if injectors may cause this issue? Mine has done 250000kms and it has never had new injectors. I heard they may not last that long.
Anyone have an opinion on this?

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Tony
76 W116 280SE
74 W107 450SLC
88 W124 300E
82 W123 280TE