Have searched forums extensively here and elsewhere in attempting to troubleshoot issue. 420SEL starts immediately, but idles really rough for approximately 10 seconds before smoothing out. Behaves the same whether it's started after sitting overnight or after warmed up and then re-started after 10 minutes or so. Based on fuel accumulator reads it doesn't appear to match this scenario and no vacuum leaks were detected based on gauge.
I've replaced cap/wires/rotor (originals with 87k m) and put in non-resistor NGK's. This improved idle which is smooth when warm although occasionally I also have a slight rough idle only when in gear. Engine under load runs exceptionally smooth with no indication of miss or roughness felt.
Possible related issues - in D or R engine idles at 500 rpms (seems low) also I have a slight, but consistent tick coming from passenger side valve cover leading me to think I may have a rocker arm, shim or accumulator issue. Not sure if there is any way to check clearance on rocker arm without the special tool.
Thoughts on what to check first for temporary rough idle?
I would do a fuel pressure leakdown test with a gauge. I'm thinking one or more injectors are leaking after the engine turns off, or the seal at the bottom of the control piston in the fuel distributor is leaking. Check the latter with a strong light and mirror to look under the air flow meter flap for puddling fuel.
You might also disconnect your cold start injector when the engine is warm and see if the problem persists under that condition.