My Bold 9790 keeps rebooting.

Hi, I bought a used BlackBerry 9790 smartphone and it's experiencing a few problems. I really love my blackberry and there is now way that I can return it back but I can only get it repaired, Here are the problem :

1) Automatically turns off by itself
2) Turns off itself after using flash 1-2 times
3) When the battery is fully dead a X mark appears
4) Restarts by itself and battery is drained
5) Restarts when a new sim card is inserted

These are the problems I am experiencing but my new smartphone is like brand new, no scratches and the body is like new. I also tried installing a new OS and but the same happens after using flash : it turns off.

Is there a problem with the battery or the hardware ? I guess the battery is weak and what should I do ?? Please don't tell me that I should return it back as I cannot.

Thanks.

Welcome to CrackBerry! Have you tried replacing the battery? Maybe SDCARD is damaged?

Replace battery with an official RIM/BlackBerry new one, then upgrade the OS.

Thanks for the replies everyone, I reinstalled the OS but nothing happened. Then I went outside an bought a new genuine battery with a warranty from a retailed seller and he gave me a chance to test it as well ! I fired the flash around 15-20 times and nothing happened, the smartphone is now working like a charm ! Thanks for the replies people, appreciate it.