1979 245dl with b21f/m46

Hello

The car sometimes would stumble and lose power to about half on the highway, but the rpm never even moved but it always corrected itself until recently now the car will act like that on a daily basis.

Now it will even do it at idle and once it started happening while the hood was up and i could see it happen.
The frequency valve is turning off right before it happens and comes right back.
It is not ridmic and it is random so.
Could i just assumed that the valve needes replacement or is it turning off because it is being told to?
Is there a way to check?

Thank you

all the best
Antonio