After a great deal of messing about I finally found out what was wrong with my MacBook. One of the sticks of RAM was bad. It's from Apple, and I'm under warrenty so that's not a problem. My question is do I need to take the whole MacBook to an Apple store, or can I just post the RAM somewhere and they'll send back one that's good? I'll be giving them a ring when I've got time, but it'll be nice to know before hand so I can ask for it.