Not sure about the 6-cyl, but the 4-cyl 190E has easy timing adjustment by twisting the distributor (undo the allen-bolt). I can't figure why your performance should change at times, and the knocking is only occasional (unless you're changing fuel), could the allen-bolt be lose? Sounds unlikely but worth checking & tightening.
You should cure the knocking first. If you don't have the oscilloscope & equipment for this, timing adjustment is easily done by any half-competent tech. From here I guess you can then see if you still have poor performance.
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black