If like mine (until this morning) has been exibiting the behaviour of shutting itself down (with no prior warning) after 15 or so minutes running on battery power (but is fine when plugged into the mains); it could well be a battery problem. I took my (6 months out of warranty) machine into my local Apple Store this morning, was given a replacement battery and am now happily running on battery once more.

SteamSHIFT out.