System fault diagnosis, please help!

[hamhamham32]

Hi!
Can you please help!
Basically my system crashes undergoing basic tasks such as web browsing. It doesn't need or seem to be hot for this to happen sometimes only get 30 minutes basic use out of the computer before it happens watching videos on youtube or something the sound from streaming videos carries on playing for a short while after it freezes before going in to a loop or just producing noise, sometimes the screen goes black or it will freeze whatever image is on the screen before changing to something glitchey and then it just goes black. The computer doesn't seem to respond to alt f4 commands or anything and needs to just be switched off at the power button.

Does anyone know how I can diagnose and fix the problem, or know what these symptoms are? Its basically rendered the computer useless and I need to know what needs replacing or if there is a simple fix?

Many thanks in advance.

System info:
Intel i5 - 2500k 3.30ghz quadcore
amd radeon hd 6800 (not been overclocked)
gigabyte z68x-ud3p-b3
windows 7 64bit
8gb ram
cant remember what PSU i have think its 500watt

 

[stulid]

You need to find out what PSU it is.

After that I personally would,

Update the BIOS
Reinstal windows+drivers

 

[Gunda]

If what stulid above doesn't fix it try disconnecting and reconnecting all power leads, and reseat the graphics card too.

 

[Snodge]

Reset RAM, and reset CMOS aswell

 

[hamhamham32]

ok i'll try that, many thanks!