Sounds like a dodgy coil pack connector or one of the wires going to it. Mine has the same problem but it only happens when I disturb something. It goes down to 5 cylinders, so I try to find out whats wrong, then after about 5 mins of me unpluging/plugging connectors and wiggling wires it just goes back to 6. Usually when I've just about given up.
From memory the 3 wires going to the coil packs are +12v with ignition on, permanent earth, and the other one runs to the left hand PTU connector and tells the coil pack when to spark. The earth and 12v are easy to check, but the other one a bit of a pain as you have to remove the connector off the PTU.
I checked all of mine and didn't find anything wrong, but it was running on 6 at the time so that makes sense. Also, try checking the pins in the coil pack connector, to make sure they're not shoved too far up or or bent apart or something. Never heard of this but you never know.
That's alll I can think of at the mo.
Rob