advice on why my xbox wont load my back ups (somtimes) pls

hi i got a xbox 360 off a m8 which has been flashed(so good so far). but my problem is when i turn the power off at the electric socket. then turn it back on when i wish to use the machine, i have to reset the clock. i have a hard drive and have set up an user account. i also have to do this to play my back up games, some times when i leave it swiched on at the socket but shut it down over night i have to wait a few mins to load a abck up. the xbox starts up ok and goes stright in to my user account but when i put a back up in it reads it but nothing happens. then after a few mins i try the same back up again and it loads ok. pls help. its my 1st x box