Computer randomly crashes?

hello i have a custom built pc, here are the specs
gigabyte p55-ud4
i5 lynnfield
asus eah4890
4gb corsair dominator ddr3 1600
antec truepower 750w

rest is sort of irrelevant

so the real problem is my computer randomly crashes after or while playing games

its not like there very intense games, its only like l4d 2 or assassins creed
when the computer crashes, it doesnt really turn off
the monitor says no signal and the sound goes all weird like
also the fans on my gpu go up to their max rpm

and itll stay like that until i do a hard shut down,
then itll be fine for the next few hours

any ideas on what the problem might be?

Check your games settings. It may be too high. Is your video card OC?

well i usually overclock it when i play crysis w/ asus smart doctor

but when im playing regular games i put it back to stock

i havent played crysis in about 2 weeks and it just started 2 days ago

other than that nothing else is ocd