missing a Cylinder? :(

[Gaz]

3 times have i been driving my car when suddenly the engine starts to stutter and shake and loose power.

On tick over its very rough, and while driving its absolutely awful.

Its as if its only firing on 4 cylinders (maybe less) so it could be an ignition problem.
OR maybe its fuel related ?

The 3 times its happened, it seemed to clear up when i take it to 4.5K and higher.. but dropping back down the revs and the poor running returns.

I couldn't work out what was wrong, but a full bore, high rev Italian style 'tune up' down the bypass seemed to fix the problem. LOL

Anyone got any ideas ? Why is this problem coming and going?