OFF & ON Repeatedly w/ Restarts & Blue Scr

Hello,
my computer turns off and on, once my screen saver comes on (10-15 mins.) or has been on for over 3-4 hours and goes to blue screen before it restarts, repeatedly. i have to push F8 in order to put in safe mode or reboot or use the "last known configuration" option but doesn't always work. if it does it once, most likely it repeats over and over, until i just turn computer completely off for an hour or so. any advice would be appreciated. thanks!