Currently Being ModeratedJan 19, 2013 2:44 PM (in response to sadiethep)
The symptoms are consistent with your battery reaching it's end of life. You can replace it yourself if you don't mind some fiddly small work, or pay Apple or another 3rd party to do it for you. My original 2009 iPhone 3GS never experienced the unexpectected shutdown issue, but by last summer it's battery life had fallen to about 2/3 what it had been when new. I bought a new battery for $20 from iFixit and gave my son the newly batteried iPhone 3GS once the 5 came out in the fall. Has worked like a charm ever since.