1998 V70 Rough Idle/Misfire/P0305 After Long Time Idling

[bubbeck]

Hey volvo wizards,

I roadtripped down to NYC from Boston this week for work, and my 1998 V70XC with 222k miles drove great the whole way down. However, while working in the city today, I idled the car for several hours continuously to charge some electronics/tools etc, and I noticed that over the course of the day/hours of idling the car began having an increasingly rough idle (RPMs dropping from 1000, sputtering sound from exhaust.) At the beginning, I just thought the drop in RPMs was from the alternator kicking in to run the chargers, but by the end of the day, it was so bad I was worried about the car stalling. When I checked the codes, I found a new P0305, cylinder 5 misfire and cleared it. Once I got it moving though, it seemed to run fine on the hour-plus trafficky drive to where I'm staying and didn't have a new P0305 code.

So two questions:
1) Any idea what might be causing this? I recently replaced and gapped all 5 plugs (not the cap/wire/rotors though - those were done ~40k miles ago). I also recently replaced the timing belt, and have done the PCV in the last year. I also recently did a compression test (before TB job though) and got good numbers on all 5 cylinders. I've also had a P0440 evap code for years now that I haven't bothered fixing if it might be related.
2) Can I let this ride for the drive back to Boston where I have a garage with tools, or should I deal with it ASAP? I put some techron fuel injector cleaner in the tank, hoping that helps.

[abscate]

Have a good look at the cap rotor and plug wires focussing on number 5 cylinder of course.

Idling all day isn't the best thing for an old engine, pull off the idle air control valve (IAC) and check for cleanliness too

I would drive it back to BOS if it's running ok at revs

[jtp]

Could just need a hard driving Italian tune-up to get everything lose.