Unusul behavior from a 12" PB

I have a 12" 1.33 ghz PB g4 that has been acting strangely lately. While still having around a 40% charge, it just shuts off for no reason at all. No low battery warning or anything. It just shuts off. It won't turn back on again until I've plugged it back into the mains. Pressing on the battery indicator doesn't do anything either. Once back on, it is still around a 40% charge. It shows this both in software and on the actual battery itself. I've reset the PMU without any luck. Anyone have any ideas?