Why does my computer freeze?

[Conordh]

My computer freezes completely forcing me to hold down the power button to restart it also makes a sound loop noise which sounds like a buzzing noise when it freezes, this only happens when I play certain games and I'm not sure why.
The main games that freeze the computer are DayZ (Arma 2 mod), Battlefield and for some reason Candy Crush on Facebook, however it doesnt seem to happen when I play Minecraft or World of Warcraft.

The temperature seems to be fine whilst playing, I have done multiple virus scans and nothing has come up. I have an overclocked CPU but I've seen that fixing that still doesn't work for other people. My graphics drivers are also up to date and I have tried installing older versions back to when it didn't freeze and that didn't solve it either.

Here is my system spec and temperature with just this page open:
Operating System
Microsoft Windows 7 Ultimate 64-bit SP1
CPU
Intel Core i5 2500K @ 3.30GHz 40 °C
Sandy Bridge 32nm Technology
RAM
8.00GB Dual-Channel DDR3 @ 665MHz (9-9-9-24)
Motherboard
Gigabyte Technology Co., Ltd. Z68AP-D3 (Socket 1155) 39 °C
Graphics
VP2365 SERIES (1920x1080@60Hz)
Intel HD Graphics 3000 (Gigabyte)
1024MB GeForce GTX 560 Ti (ASUStek Computer Inc) 35 °C
Hard Drives
466GB Hitachi HDP725050GLA360 ATA Device (SATA) 34 °C
Optical Drives
DiscSoft Virtual SCSI CdRom Device
DiscSoft Virtual SCSI CdRom Device
HL-DT-ST DVDRAM GH22NS70 ATA Device
DiscSoft Virtual SCSI CdRom Device
DiscSoft Virtual SCSI CdRom Device
Audio
High Definition Audio Device

Help would be much appreciated, thanks.

 

[Tabris DarkPeace]

Are you overclocking anything?
What PSU are you using and how old is it?

When did the problems start?

 

[Conordh]

Are you overclocking anything?
What PSU are you using and how old is it?

When did the problems start?

The CPU is overclocked and possibly the graphics card, the computer is a custom built one and I bought it from a family member already overclocked.

The problem started about a year ago, I have done a fresh re install of everything on my computer and it allowed me to play games for a bit longer, still not DayZ though, the the problem has gotten worse again so it wasn't really sorted.

The PSU I am using is a 650W corsair, not sure how old it is sorry.

Thank you.