2000 V70 SE, 268K miles, AW-5, Denso

Car has developed a symptom of rough running immediately after cold start. Severity seems to be proportional to how long the car sits unused.

After one day of sitting cold (mild climate), it's a minor startup shake. But yesterday, after sitting 10 days, the engine shook and vibrated so much I was watching for the CEL to illuminate; which it did not. Rough running clears up in 30 seconds at cold idle and runs OK for rest of the day.

Car has a history of #3 coil misfire, which was tracked down to a bad coil receptacle housing and wire terminal. There have been no #3 misfire symptoms for about one year now. When chasing that misfire problem, the Indy tech said I was using the wrong plugs (NGK copper core) and replaced ($$) with NGK platinum plugs. Those platinum plugs have 13K miles on them now.

My first thought is replace with new plugs, back to copper core, and see if this clears up. Any thoughts?

Thanks,