Computer keeps crashing? :(

Hi.

I recently got a second 6950 card to run in crossfire with my old one. I have the power requirements and everything so I know it's not because of that. A while ago, I used to play Maplestory, and once in a while my computer would crash; it would freeze and there would be a buzzing noise coming into my headset, like it was stuck on the last sound that played before it crashed. This never happened when I played League of Legends. The last few days, it's been happening on League of Legends, too. Why is this happening? I can't afford to crash while playing League. This is really frustrating and worrying. Could someone help me please?

Specs:

Powercolor Radeon 6950
Asus Radeon 6950
XFX 750W Core Edition 80 Plus Bronze
Asus P8Z68-V/GEN3 Motherboard
Intel i7 2600K 3.4 GHz
Intel 160GB SSD
WD Caviar Black 2TB HDD
16GB Kingston Hyper X Ram