Computer Freezing

[ManThunder]

Hello everyone I really need help, my computer is having some problems, whenever I'm on my computer after about 5 to 10 minutes the sound cuts out and then it completely freezes but i can still move my mouse, to get it to work i have to either press the reset button or turn it off at the wall or from the power supply, then it would be fine for about an hour or longer then it happens again. It has never done this before and i have had my computer for about 6 months.
If anyone has had this problem before and knows how to fix it please tell me, I've done anti-virus scans and nothing has come up. I really need help.
Thanks

[DaveLembke]

I'd run memtest86 on this to make sure its not a bad memory stick.

Does this always happen when running a specific task or randomly?

Also what is make/model of computer?

[ManThunder]

Its happens randomly in game and just browsing the internet.

My computer specs are:

Operating System:
Windows 8.1 64 Bit

CPU:
Intel Core I7-4770 3.40GHZ

Mother:
MSI-Z87-G45 Gaming Motherboard

Power Supply:
Thermal Take Smart 750w 80 Bronze

Graphics Card:
Nvidia Gainward Geforce GTX 760 Phantom 2GB

Ram:
Transcend 8GB (1x8Gb) 1600Mhz

But I'm getting new ram

Kingston HyperX 16gb (2x8gb) at 1866Mhz