Windows 7: Crashes without the blue screen

Hello once again. I posted a thread - hard drive failing? - a few weeks ago and i'm still having problems. Some one suggested that I run a test on my hard drive but I was unable to find a test to run for my OCZ drive.

The problem i'm having is that after I start my computer and it's been running for a while, all of a sudden the screen gets dim, then goes blank. Then it comes back up with no icons, just a screen with the Windows logo. Then it goes into the Windows error recovery screen and i'm able to start windows normally... Sometimes.

Sometimes i'm unable to boot up and the screen comes up with an error saying "cd-rom boot priority...no medium." When I restart and press F11 to go into the boot devices list, the drive that my os is on is not present. The drive doesn't show up in my BIOS either. So for some reason my drive disappears and is unable to boot my os. Can anyone help me with this? Is this a hard drive problem or a mobo problem?