This is a freshly built machine, and the problem manifested during windows install, so it is a hardware problem.
The machine works for some hours, but then restarts without warning. First, it restarts without warning, but with a complete shut down - windows closes all programs, displays the "windows is shutting off screen" and so on. After the restart, the machine survives a shorter time, turning off more abruptly. By the third or fourth cycle, it barely makes it to the boot screen before restarting. The only way to break the cycle is by cycling the power supply. Letting the computer sit for a while, restores it to the previous state.
The problem occurs at times that are sometimes consistent - always at 6:45 am, system time, but others too.
This seems like an overheating issue, but checking temps on motherboard, CPU, and Video did not show anything weird. I suspected power supply issues, but an identical replacement did not solve it. Resetting CMOS did not help.
Disconnecting DVD player+HDD did not help (the computer started restarting from BIOS.
The specs of the machine are:
COOLER MASTER GX Series RS650-ACAAE3-US 650W ATX12V v2.31 80 PLUS Certified Active PFC Power Supply
ASUS ENGTX570/2DI/1280MD5 GeForce GTX 570 (Fermi) 1280MB 320-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
Seagate Barracuda XT ST32000641AS 2TB 7200 RPM SATA 6.0Gb/s 3.5
LITE-ON CD/DVD Burner - Bulk Black SATA Model iHAS124-04