Alu Macbook hibernates at random instead of sleeping

I've got one of the original Alu MBs and since SL its started an annoying habit. A lot of the time, whether on battery or power, instead of just going to sleep, it goes in to hibernate mode. It doesn't matter how long the machine is left for before trying to wake, or how much battery is remaining.

Any ideas? I seem to remember this was a problem with the machine just after launch as well, but OS X updates fixed the problem. Now its back....

Thanks.

 

Will that let me disable hibernate mode? I don't even need to shut the lid, I can just walk away for long enough for the screen to dull, come back and its hibernating.