The EZCool 550W PSU is a £20 low quality unit, not suitable for your graphics card. It only provides 24A on the +12V rail (see the label on the side of your PSU). A good quality 550W PSU should have 40A/+12V and will cost about £60-70. You need to upgrade.
What are the temperatures and voltages in BIOS and while running a fullscreen game?
Download and run these 2 programs:
(go to the Computer>Sensor section to see the temps/volts)
(click the Sensors tab to see the GPU temperature and fan speed)
Leave Everest and GPU-Z open on the desktop and run a fullscreen game. After about 10 minutes, hit Alt-Tab to return to the desktop, then hit the PrtSc key to take a screenshot. Post back with the screenshot so we can compare the idle BIOS readings with the stressed Everest/GPU-Z readings.
How are your 5 case fans positioned? Too many fans can cause turbulence and increased system temperatures if positioned incorrectly. You only need 2 case fans, one at the front pulling in cool air and one at the back blowing out warm air.
How long did you run memtest for, and did you test each stick individually by removing all the other sticks?