PC reboots while im ingame

Hi, my pc sometimes reboots while im ingame, just like it wants to. i have this problem with serveral internet games. does any1 have a clue how to fix this? i already changed the graphic card but no change :-/

do you have a hard ware monitor. i had a box that would do that and the 5volt on the ps supply was droping below 3.5 volts.

you RAM memory can be instable. BUT it can be virus/spyware so do a reinstall.