misfire under light load

[200q 20v]

Fellow experts,

Driving in 4th gear, 2000 rpm, increasing throttle feels like a
misfire in several cylinders. E.g. virtually no power.
It is not very pronounced when engine is cold.
It appeared to start after last track event. Leaving the track I
noticed slightly rough idle and above mentioned misfire while driving
home.
Changed several years old spark plugs (found the cheapest F5DPOR on Amazon).
Virtually no changes. There are no vacuum leaks (cold engine pulls
full boost with no problems) no codes in ECU.
Strangely enough during recent heavy rain misfires were much less pronounced.

If left on idle a few misfires a minute can be heard.

Wires and distributor cap are about 5 years old, much less then
average miles per year.

I wonder if anyone has any ideas. I plan to look at replacing cap / rotor next.

Eugene.