What is causing system hangup?

My customer has a newly built system running Win Server 2003 Standard x64 R2 Sp2, over the weekend when he comes in to use it it will be hung up. He has to reboot several times before it comes all the way up. Once up it runs great till the next weekend. What could be cause the hang-up? Why does it need several reboots before it is successful?
I7 950 @3.07Ghz
8gb ram
Gigabyte X58A-UD3R mobo

Any help would be greatly appreciated