For the last few months my phone would start shutting down if I used the internet or the camera. Now gradually it's gotten worse, if I leave the messages screen open for a few minutes it will shut the phone down. When I turn the phone back on it will either shut down immediately after, or it might turn back on and the battery will be completely drained (even after charging it to full), or the battery might be back to the level it was originally at.
Basically my phone is becoming more and more unusable by the day. I did a factory reset yesterday and while that made me phone run so much smoother it appears it didn't help fix this problem.
My initial guess was that the phone was overheating (it was quite laggy as well) but now I'm starting to fear the battery is just gone. However this doesn't make sense because my dad and brother also got the exact same phone at the same time I did (summer of 2010) and they don't have this problem. And it bugs me because I have taken much better care of my phone than them.
What else can I do at this point?
Basically my phone is becoming more and more unusable by the day. I did a factory reset yesterday and while that made me phone run so much smoother it appears it didn't help fix this problem.
My initial guess was that the phone was overheating (it was quite laggy as well) but now I'm starting to fear the battery is just gone. However this doesn't make sense because my dad and brother also got the exact same phone at the same time I did (summer of 2010) and they don't have this problem. And it bugs me because I have taken much better care of my phone than them.
What else can I do at this point?

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