The car would start stuttering as it warmed up but only when the engine was under load (and then at any throttle position). At first it would clear at 2500RPM, then 3000, then 3500 when it was fully warm.

I got home last night and checked the old plugs--they were normal. Ceramic was a little yellowed, the electrodes were charcoal colored, but definitely not sooty.

Put grey RTV (liquid gasket) on the corners of the valve cover, and R/R both the sparkplug wires and the PCV valve. The PCV valve was definitely gunky with oil and when shook, it felt/sounded nothing like the new one that went into its place.

Problem solved!