When my 95 S420 (sold) misfired under hard acceleration it was bad plug wires.

Replacing the coil plug wires is difficult enough. Replacing the coils is downright impossible!

I noticed that one coil wire was touching a bolt just below the driver side distributor. Routing the wire properly made an improvement but the problem didn't clear completely until the wire set was replaced.

I'm not sure watching plug wires in the dark is conclusive. M119 plug wires are routed in tight channels. Cross-firing might not show in the dark. Still worth doing, IMO.

Sometimes these engine react adversely to platinum plugs. Check that you have the correct copper plugs. Bosch Super F8DC4, I believe.

Sixto
93 300SD