G'day

I have a small missfire/(surging probably caused by the miss) at light throttle at most speeds. I feel it especially at motorway speeds of 60mph etc.
Under full or heavy throttle its as smooth as a babies skin.

I have replaced the fuel filter today but that made no diff...I'm a new owner of Mercedes and am comfortable with mechanical repairs.

Any advice?

1989 300E
170,000km
Full service history


Thanx in advance