If the SAPPHIRE HD 4850 graphics card is compatible with Windows 8 you can test it to see if you get the same issue or not.
If the issue persists you can try another cable or monitor if available.
You can run a test to the processor using the following utility which will stress the features of the processor to see if those are working fine or not http://www.intel.com/support/processors/sb/CS-031726.htm
Another troubleshooting step you can perform is updating the BIOS of the motherboard. You can find BIOS updates in the following link http://www.asus.com/Motherboard/P8B75M_LE/#support_Download
Been there, done that!
I have already installed the latest bios update firmware and run the test processor program with success.
Also, I have installed the 4850 card and the problem went away!
But I will not use the 4850 in this system so I cannot continue like that.
Cable and monitor are not an issue, as well.
I recommend contacting the motherboard manufacturer in order to check if the issue is being caused by the video ports on the motherboard or not. You can use the following link http://support.asus.com/contactus.aspx