'04 Model running on one cylinder after a short ride

[CreepiestRaptor]

I've been having an issue for the past 2 months that I can't work out what is the cause.

Occasionally my left (as you're sitting on the bike) cylinder cuts out and I don't know why.

I have replaced sparkplugs, leads, caps, coils, battery to make sure it isn't electrical (They kinda needed changing anyway)

If I'm riding about and my revs don't go above 5500rpm then everything is fine as I can go to and from work like this without issue. But if I run higher then the bike starts to judder and then the left cylinder cuts out and is difficult to start it back up again.

Anyone got any ideas?