Need help on X79 build!

I finished my first computer build yesterday with the following specs:

Coolermaster HAF X
Asus P9X79 Deluxe
Intel i7 3820
Corsair H80
Corsair 850W AX
Corsair Dominator 16GB 1866MHz
Corsair Force 3 120GB
XFX 6950 2GB XXX
LG BluRay RW Drive
Windows 7 Ultimate x64

The computer installed windows perfectly without any problems, and there are no problems when booting into windows. The problems arise after using it for a couple of minutes. The problems are:
1 - the screen foes blank for a second and then back to normal
2 - the screen goes blank and eventually the computer reboots
3 - the computer completely freezes (the mouse, wifi, and everything else stop working) and the computer has to be shut down by force (reset buttons stop working and power button has to be held)

I have no idea what is causing these problems; the drivers are all up to date, bios has been updated and the connections have been checked. Is this a hardware fault or have I not configured it properly?

Also my external USB hard drive is not recognised by anything in the computer (not the bios or windows and I have tried every port) and I have checked that is does work on every other computer. What is causing this problem?

Thank you

Welcome to the Forum! http://www.ocforums.com/showthread.php?t=491697
What are your temps showing in the bios?
Did you set the AI tweaker on auto or manual?
Check your ram settings also.
Maybe go into the Intel board and inquire on settings cuz I don't have that chipset yet...

Did you make any changes in the SATA configuration after installing?
http://www.techpowerup.com/reviews/ASUS/P9X79_Deluxe/images/bios_twentyfive.jpg

The bios is showing temperatures between 25 and 30 degrees.
I haven't changed any bios settings yet, the default for sata is the same as the picture you posted.
My RAM is working fine, 16GB running at 1333MHz

I have fixed the main windows issues by changing the sata connectors from the Marvell controller to the intel x79 controller on the motherboard.
But the external hard drive issue still exists, and I have tried to download drivers but the only support windows 98/ME. What should I do?

Check to see if your USB drivers are up to date. The last update I see was on 2011.12.16 with a version # of 1.14.3.0. What kind of external hard drive do you have? Have you tried another USB cable?

I have the latest USB drivers and the USB cable works on other computers so I don't think it can be that........this is really puzzling me as there is a device in device manager called unknown device, and it can never find the drivers

There might be a setting in your BIOS to allow USB 2.0 compatibility on USB 3 ports.