T7600 Shuts down and turns on randomly

I have a T7600 with a 2687w processor... I use it for rendering video, primarily.

A couple of weeks ago, in the middle of a Premiere session, the Windows (7 pro) screen for shutting down started popping up.  This is/was not initiated by me.  Fortunately, the software I typically run (CS6) prevents the shutdown from happening - as you're prompted if you want to save before shutdown.  If I'm not running similar software, it will just shutdown.

The shutdown process is normal... it's not like the computer just goes black.

Then... the computer turns itself on... without me pressing the power button. (this is not immediately... at times it's hours).

This is not a WOL condition, as I've turned that off, and even disabled LAN in the BIOS.

I have also disconnected my graphics card and booted, and it will shut down after awhile.  Same goes for the PERC 310, and all external devices (disconnecting them).

Instances of this are becoming more frequent, and all random, there is no regular interval.  When it first happened, I would just cancel the shutdown message and continue working... sometimes hours at a time.  Lately, it's become a pretty cyclical process... usually within 10-15 minutes.  Because of this... I'm sort of doubting what I thought originally (a short).  Could this be a PSU issue?  Would a bad PSU cause the device to go into shutdown and turn on by itself?  It's not the power cable, either... I tried a different one.

I cleaned this thing from head to toe recently, ruling out dust.

Checked Event log... and there isn't any info other than the 1073 requests (shut down cancelled by user).  Diagnostic lights on front aren't helpful either... they are operating normally.

Reset the BIOS by pulling the coin battery... no improvement.

What it is NOT:

1. Bad PSU power cable
2. Wake on Lan option
3. Connected Peripeherals
4. Dust
5. does not appear in Event Logs (outside of 1073 messages)
6. GPU
7. Surge protection/power outlet issues.
8. BIOS on power down issue
9. User event based (HID's like keyboard and mouse)
10. bumps and dings (like moving the unit does not cause it... and the machine is in good shape)
11. I even disconnected the front panel to see if the switch was sticking... it is not (to my knowledge.
12. Viral or malware.  Has never been connected to internet/LAN.  Only used as Offline machine

When it reboots on its own, windows loads normally (as it goes through the shutdown process each time it shuts off).

PLEASE help.  I have searched what feels like a million message boards, with zero luck in finding a solution.