Computer turns off/ restarts while playing games?

This has been happening for a while now, I have also tried opening my case up and running games such as sims/ star wars the old republic and it still happens, I have felt everything in my pc and nothing seems hot.
It does this during most games i play, sims 3, sim city and star wars old republic. I have a Radeon HD Graphics card, 8gb ram, 500w PSU, 6 core AMD processor. Does anyone have any ideas? The pc just turns off with a black screen so I do not get no blue screen or anything. any ideas? Thanks a lot.

Also -
I have tried scanning for viruses/ malware etc and i do not think its that.

 

I just ran furmark and the computer basically turned off straight away? :/

here are some specs
http://i368.photobucket.com/albums/oo125/joeycookson/213121_zpsfbb093f1.png

 

Welcome to MG's Joey!

Which Radeon exactly and what is the make and wattage of the PSU (a photo of the PSU label would be good)?

Furmark basically tries to burn out the gfx card - in a very controlled sort of way - heat builds up fairly slowly (on a well-cooled system) but it's certainly not a software to start unless you're sitting in front of it, watching the temperature stats!

 

i used to have something simular happen to me with an early 2003 gateway e500s pc i used to have it would sometimes suddenly do a hard reboot while playing a pc game (i pretty much maxxed out my psu with a nec dvd rewriter plus ether nvidia gefore or ati radeon agp card over the years i had that pc i upgraded to one or the other plus my psu was about 400 watt) so you might be putting to much of a load on your psu causing it to shutdown and restart your pc.

 

So you think the chances are it is a psu problem? What are psu's like to replace? Because I see a hell of a lot of wires coming out of my psu so i can see it being a pain in the ***

Oh some more specs.
my g card is a Radeon HD 6850
http://i368.photobucket.com/albums/oo125/joeycookson/13DD9BCD-8183-4FBA-8718-3BE7B6DEF4D9-16104-000006F1CA98FF64_zps4d019af9.jpg
is my psu sticker

 

That PSU would be top of my list to take the blame; no telling who actually made the unit, it could be a reasonable make with decent components, it could be a rubbish one. See the varying comments here for example: http://forums.hexus.net/pc-hardware/192903-novatech-powerstation-psus.html

The best brands are Seasonic/XFX, some reasonable brands like BeQuiet, many other brands use several different makers through the range and even the lifespan of a single model.

First check the gfx card is seated correctly, then check the power connection on the rear of it, or swap it with the 2nd connector of that type and test again. If both connectors fail, don't risk it anymore - bad PSU's do kill other components - buy a decent one, swapping them out is pretty straightforward, 20 minutes maybe (but it may take a lot longer to route them neatly).

 

Ok everyone thanks a lot, will buy a new psu soon then, i think ill go for a 750w psu. Should be able to swap them fairly easy just doing 1 wire at a time like advised below Thanks a lot.

 

Just don't go for wattage, go for quality.
In, my some what humble opinion. Go for anything that is gold or higher quality. I'm a platinum kinda guy. Yes, it costs more, but, I'm good to go.

Decent Gold quality: http://www.newegg.com/Product/Product.aspx?Item=N82E16817151087 $150

Or better Platnium: http://www.newegg.com/Product/Product.aspx?Item=N82E16817151120 $180

 

Amperage and voltage are an inverse proportion.

I'll take under powered and heat for The Daily Double, Alex.

The lower the voltage the more the amperage draw. Amperage is heat, And it will shut down your system.

 

in my current gaming pc (a custom build) i put in a rosewill green series 630 watt the amperage is 50 plus i've had it for about two years and never had any problems with it (rosewill is a good brand) mewegg no longer sells this psu but amazon has it http://www.amazon.com/Rosewill-630-Watt-Series-Certified-RG630-S12/dp/B004GL8UO2 plus removing a psu is not that hard to do some power connectors can be a pain to unplug but they will unplug just don't pull om the wires.