Having thought about it for longer now, it could be coilpack-related... check your coilpacks and/or spark plugs while you're in there. If they're still the old stock coilpacks in there, it's quite common for them to start failing. There's a DIY method of siliconing up the cracks in the edge of them to help get the spark where it's supposed to go.
Failing that, you would need new (or secondhand from wreckers) coilpacks to replace the faulty one/s.
Could also just be dud plugs.