Any time of day or night.
Every day or two at random times.
Whether or not connected to the Internet (I tried pulling out the Ethernet cable at night).
No power surges - and the PC is on a business-grade UPS.
MalwareBytes Pro, ESET Smart Security, and Kaspersky Virus Removal Tool find no malware.
There are 8 x 3TB data drives on the PC, so scanning takes a while!
All installed software is legit.
Initially the OS was Windows 7 Pro - the PC did not self-start then.
Then I upgraded to Win 10 Pro - and the problem began.
A recent fresh install of Win 10 - the problem persists.
Any suggestions on how to investigate this further?