What could cause a server to lockup?

My server is freezing at intermittent times after a few minutes of booting to the desktop, even when opening my computer, it freezes/locks up and I have to reboot. After several reboots, it worked for a couple of days, then the same problem occurred. I see nothing unusual in event viewer. I thought it could be a ram problem, but after running the server fine in safe mode, I doubt it’s a ram problem. I scanned it with A-Squared malware scanner; clean scan.

I also ran disk cleanup, I thought the server was bogged down with files. It still freezes after a few minutes when booted to the desktop.

I’m at a loss.

Any suggestions?

specs: Windows Server 2008 Single domain

-Windows Memory Diagnostic Tool reported no errors.