miss at 1800 rpm ,

tune up was cap ,rotor,plugs, wires, pcv, air filter, fuel filter.clean crankcase with seafoam. the cat is rattling from inside the case?

you could have an injector with a bad spray pattern. you can get an injector service from a shop for a reasonable cost as replacing the spider assembly can get very pricy. I would try this first. Good Luck Roy

just installed new cap , and miss did go away for about a week