it used to happen to me when i was playing city of heroes, the reason was that my video card got to hot so the computer was shuting down and that aggrevated me alot, don't know if thats the case for you, maybe check if the card is hot when it happens again
i like your work they look good, for the mp3 player i found that the menu, play, pause, ect was too light, i really love the sharingun or whatever its spelled eyes from sasuke looks awesome
like everyone said its a bit dark and if the guy could be just a tad lighter, and the white wavy things looks like sperm to me lmao i would give it a 7/10 good work