Next time it happens take a plug out and check if it's sparking and what condition it's in if it's wet etc
Can't see why it wouldn't be, as said once started it runs fine.
I chopped the ends of the HT leads and rescrewed the caps on firmly, got new Iridium plugs, and i've covered them in WD40 as well to avoid damp issues.
Don't forget, this isn't a recent issue, it's been happening since summer so i've done all the main checks and replaced a few parts along the way.
The air vent theory was only thought of when i was searching the web a few days ago, but i've got this week off work so not been doing the commute.
I was out on the bike yesterday but i'd already opened the fuel cap to blow air down the hole, plus the weather was warmer, and the bike started up fine straight away.
The main time it plays up is after it's been sitting outside work all day as i'm going home. It's played up a few times in the morning but not as bad, although it is garaged at night so that may be why.