iPhone 4 turns off on its own and doesn't go back on until plugged into pc/power

my iphone 4 started doing weird thing of sudden.
first day: turned off on own , wouldn't go on when pressed 2 buttons or top, or reset (if that's it's called), button. have go on when plugged power source. battery full during this.
second day: turned off 3 times , go on when plugged macbook or power source.
today has been doing on , over! reason? has battery gone mad?
please help! , don't tell me take apple replacement! have no warranty , want know what's wrong before decide throw away.
had ios 5.0.1 on it. thought maybe updating fix problem did not work. can't go previous version because iphone has never been jailbroken.

 

it sounds either battery bad or it's not connected anymore. make appointment apple genius , have them check out. if need battery replacement think it's $99 or something.

 

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