Random USB disconnect and freezing on shutdown

I've been having a really nagging problem with my comp lately and I have essentially run out of ideas on what is wrong with it.

I've been running the PC for almost four months now, but it started to have problems a few weeks (and seems to be getting more consistant in problematic freezing/hanging).

What started happening was that my USB devices would all disconnect at once (as if they lost all power); I initially tried to just unplug and replug them back in, but nothing changed so I would have to shutdown using the keyboard which still worked (PS/2 connection). However, when I would set the comp to shutdown it would proceed normally, but the screen will sit on the "shut-down" screen and just basically hang there until I did a hard-reset with the button on the computer case (bringing me to the start windows in safe/normal/etc mode)...

I have a Logitech USB mouse and an Xbox360 gamepad plugged into two of my USB ports.

I've only gotten one BSOD randomly which gave me the 'BUGCODE_USB_DRIVER' code, but after rebooting all of my drivers appear to be up-to-date (no Yellow icons for hardware (none show in hidden items either).

I've scanned for virus'/malware, turned off the USB disable option in Windows, CCleaner, updated MOBO Bios, verifier.exe, tried to record all mini-dumps (but don't help much with no BSOD), been running 'whocrashed', run MEMTEST with 3 successful passes, operating temps have been nowhere near hot (GPU~40*c, CPU~30*c), Prime95 reports no issues, etc when the computer fails - it seems to fail for no reason (It doesn't mess up with any particular program, just whenever it feels like it).

The issue is compounded by the fact that I never get a BSOD, so being able to track what is causing the USB discconect/comp freeze has me completely baffled and out of ideas.

I'm leaning towards the MOBO chitting out on me, but at this point I have no clue at all what is wrong. Is there anything that I can do to help you guys pinpoint what is causing the issue?

Thanks much in advance