i have a nice iPhone 3GS which has a curious behaviour.
I'm not using it often but when i'm doing CPU intensive things like a game, it just shuts down suddenly, screen becomes black an i can't restart it without putting it on the charger. It's not hot.
Then i plug it, and it just boots and is runs ok with the same battery level as before the crash.
I thought changing the iOS would fix that but it doesn't .. i'm on iOs 6 and it's still the same behaviour
What could i do ? thanks