Windows 7 Random non-BSOD shut-down, weird, partial zombie-reboot

[eigenheerch]

win7 pro 64-bit, windows security essentials, newest vdefs. windows updates current. drivers current. I get these weird random system shut downs (NOT a BSOD, more like a power failure), sometimes I'm watching a video, sometimes I'm not even on the computer at all. Sometimes it happen after 20 minutes sometimes not for 5 days. no entries in the event log other than "the system shut down at [time] was unexpected".

I cannot reproduce it manually so I'm stuck trying to change something and hoping it'll make it past 7 days but no luck so far.

What troubles me is that when the computer shuts down, it starts back up again but I don't get the beep, I don't get the mouse/keyboard lights and I don't get anything on the monitor. All the fans are running, the power light is on on the tower and I DO have power on the USB ports. It almost looks like it's hibernating without the ability to wake it up.

I have to press and hold the power button, shut it off that way and then turn it back on and it boots up with the beep and resumes normally.

I tried the following:

1. Checked capacitors on motherboards, seem ok
2. Power is on very good quality power bar
3. Disabled power options (never sleep)
4. Disabled USB devices from putting computer to sleep
5. Updated all drivers

There were not recent hardware changes. Don't know what else to try. Any thoughts?

 

[eigenheerch]

Forgot to mention, I installed temperature monitoring software and my CPUs don't exceed 54 degrees Celsius. Also, video card is fairly new (18 months). fan is running and all heat synchs are clean