Something wrong with either my graphics card or PSU?

Hey guys, recently, I installed a new PSU after my old PSU - as far as I can see, was for whatever reason, making my computer turn off, and restart, during intensive activities, such as gaming or media encoding, I purchased my current PSU from a friend and as far as I know It's all in working order, however, recently, I decided to install Windows 8 before realizing that it was designed for 90 year old people who have never touched a computer, so I downgraded to Windows 7, I got my core software such as my drivers installed, Windows Update is still not complete, any way, something seems to be causing my graphics card, or something to well under perform, for example, I installed Black Ops 2 recently, strangely, it defaulted to the absolute minimum settings and 800x600 res (I think that's what it was), so as anyone would unless there playing on a computer which actually can only play games on an 800x600 res. - I changed the res. And then I started playing, the FPS was terrible, like 15 or so, noting that the graphics were still set to low. After thinking something weird was going on, I installed Brotherhood only to find, that although the FPS wasn't as bad, it was hovering around 20-40.

What do you guys think I should test? Do you think my fairly new PSU is to blame? Keeping in mind that the only other way I can "test" my PSU is by using, what I am fairly sure, is another faulty PSU.

If possible please suggests ways I can test using software, surely there is a way to see voltage to the graphics card without all kinds of PSU testers, etc.

Specs (Yes, I know It's not amazing, my I believe it should be able to play BO2 on the lowest settings flawlessly.)
i5 650.
4GB RAM
GTS 250 (1GB model).
800watt PSU.
(Probably forgot heaps of stuff, please, just ask.)

Thanks in advance.
Darkedge.