Weird squares in screen.

Hey , so i have a question regarding my computer.
When i am gaming, usually with heavier games something weird happens.
I think my computer's hardware should be able to run them but still i have problems with less heavy games aswell.
So when i am gaming i see every now and then a 'cloud of blocks on my screen , on a random place . There are little blocks showing up for about half a second , with one game for example maple story they are all black and with world of warcraft they are brown-yellowish in caves.
It is not the connection to the screen , i have tried different screens and cables.
These are my specs :
-i5 750
-4gb ram
-ati radeon 5800
-Asus p55d
-Cooler Master Real Power M520
Also , i think they are not so called artifacts , at least from what i can tell from screen shots. And Catalyst controlcenter says that the it's up to date.
I cant screenshot it since they are random, they are just little blocks on a small part of the screen , on a random place, in the same colour usally black or brown/yellowish.

It is not a performance problem.
It also happens sometimes just at my desktop screen , then they are black.
Lowering graphics in games also does not make a difference, and when i lower the resolution for example to 800x600 , the squares just become bigger.
I have attached an image ,like i said it's impossible to screenshot them, so this is what i made in paint, it looks a bit like it ,they are only there for half a second on a random place in the screen.(normally they are squares)

Thanks in advance!

Here are the results of the test.
Also when i was trying to reinstall the drivers i got a BSoD called "Memory Mangement" i think, but the second tme it installed properly.
Already thx for this straight answer , but the prblem ha not yet been solved.

Never overclocked before so everything is in default, starting to run the memtest in a minute, thanks for all this help you two =]