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I dont think this is a software update issue, but a defective issue with blackberry curve. Because I had this problem on the older 4.2 os exactly as you described, when its plugged into charging source its fine. However when doing more intense data stuff like using phone for call it would shut off.
Also when it would turn off and you had to remove battery to put it back in the battery would be substantially lower than it should be. as if the phone was sucking through the battery alot faster than it should. Mine quickly degenerated to the point where you could be on the phone only for like 1-2 minutes if lucky then it would shut off. It even got to the point where removing battery then putting it back in sometimes would get to the "T-Mobile" load up then just shut back off and didnt have enough juice to get past that screen. I am now on my second blackberry and hope I dont get the same problem. I had this and know others that had this and not have the 4.5 software or ever have.