PC monitor goes off while playing games, but computer still working in

Hey all,

Iv been having this issue for quite some time now, and not been able to find out what it is, there is many questions and answers online, but i thought id ask anyways,

My problem:
While playing games, (warcraft, fear, need for speed, dead island ect) i can be playing at any random amount of time, 20 mins, 30 mins, even an hour, and then my PC screen will go black, but i can hear the sound in the background, so i know the computer is still on, and im able to talk with my friends on Skype, but not able to see anything, its almost like my PC monitor has gone into standby, but its not the monitor at fault, as iv tested it else where.

My specs:
Asus Rampage 2 Extreme - motherboard
Corse air domination memory - Ram
Intel core I7 2.9 GHz - processor
ATI sapphire 5870 Over clocked edition - vapor x - Graphics card
850W Power supply

Graphics card Specs:
Core clock 875MHz
Memory clock 1250MHz

My specs seem a little outdated, (4 year old computer) but They are still Really good for playing games, i did have 2 monitors on one graphics card, but iv also tried going to one monitor, and it still does it (im still currently using one)

I have MSI afterburn , monitoring the CPU fan, (iv set it for 90% all the time)
And iv also tried ATI catalyst control centre (what im using now)
And still i get the black screen issue

(ONLY IN GAMES).

Thanks if you take the time to reply

More info can be added if needed (just request)