Friend has a 00' civic si with approximately 220,000 km. Stock motor, no mods what so ever.

Over the past 6 months the car has developed random cylinder misfire out of the blue.

This condition started to occur only when it was damp outside (just after or during raining) and only after sitting all night and continued until car was up to operating temperature. Once the car warms up the random cylinder misfire goes away completely.

The random cylinder misfire is occurring on cold days as well now. But does go away once the car is up to operating temperature....

The random cylinder only occurs when the car has been sitting over night. If the car is driven for a bit and then shut off...and then driven again within a few hours, there is no misfire at all..

Any idea what could be causing this intermittent issue?
ignition or fuel related...or both?? My guess is ignition related...

Spark plugs changed 20,000 kms ago to Iridium's.
Fuel filter changed 20,000 kms ago as well

 

Does the maufacture call for these plugs in the 99-2000 civic SI?
If not why were they put in there?

Also I would remove the crank pulley and lower timing belt cover, I can only assume that that high mileage motor has already pissed from the solenoid and cam seals all down the motor like all of them do.
I would inspect and clean the crank pos sensor and the counter on the belt gear on the crank.