I had a similar problem in my '82 W123 200 not too long ago. Check your fuel line, the ones that are connected to the Stromberg Carburettor, the engine will rock at high speeds if any of the flexible hoses are pinched or folded due to age, etc.
My problem was solved when I changed the damaged hose. An obvious case of fuel starvation.
Hopes this helps!
'82 200 W123