80' 450sl rough idle &

has occasional misses at idle speed and is clearly missing on acceleration. new rotor, cap & plugs. Any suggestions? I've not replaced the fuel filter, could this effect it? Seemed to start when I used a gas additive to clean the injectors.

Check you plug wires especially the metal connectors. They frequently fail and are available individually. Use and ohm meter and look for 2-4K ohms max through the wires and connector. If you get no continuity then unscrew the connector and check it. When you lulled the plugs did one or more look darker than the others? Go there first.

Thanks for the suggestion, I was suspect of them also and it matches the symptoms. All of the plugs looked good when I changed them but that's not to say that I didn't cause the problem when I changed them.
I had an intersting observation yesterday. It was the first time I used the AC in a long time and the car ran much worse with the AC on.
I'll check the plug wires, thanks.