I just built a computer for my fiancee about a week ago. It worked great for four days, then started randomly restarting, then glitching and freezing, and now it rarely gets through booting.
Gigabyte GA-A75-UD4H Motherboard
Corsair GS600 (Gaming Series 600W)
AMD A8 APU (Unlocked Lano 3GHz, Stock Cooler)
G.Skill Ripjaws 2x4GB Dual Channel DDR3
WD Caviar Blue 500GB HDD
ASUS DRW DVD Burner
Rosewill RNX-N300X PCI Wireless Adapter
When it was only restarting randomly, we couldn't find any correlation to what was running, except that they were all high-load (Photoshop, gaming, etc). So, assuming it was the APU overheating, I installed coretemp and kept an eye on the temperature while doing some work in PS and playing a few games. Temps never got above 40 C.
So I tried replicating the problems my fiancee had, and it froze and crashed. Booted up a few times normally but had problems once in Windows (more freezing). Then the display started glitching during boot and the computer was freezing. Sometimes it happens before POST, sometimes after, sometimes it loads Windows first, but usually it happens right when it starts loading Windows.
I assume that either the motherboard or the APU is malfunctioning, but I figured I'd get some info before dealing with RMAs.
If it helps, the glitching is a bunch horizontal flashing lines, always along the same lines with the bulk of it concentrated about a third of the way up from the bottom of the monitor. It's thick enough there to tell that the glitching is showing parts of a different screen, but all I can make out is "<DEL>".