My Samsung Galaxy SII has developed an annoying habit. It's got plenty of charge in the battery and then bam! It switches off. I try switching it back on and it says the battery is dead. I try again after five minutes and it switches on, it has a low battery level showing, and then it gradually gets back to the 'true' battery level. This seems to happen when I'm listening to music/radio and also playing a game or using the net. Anyone got any ideas? Do I need a new battery?