LOADING OPERATING SYSTEM or multiple restarts

[Daguin]

Greetings everyone.
I've been having a really annoying issue, which is the following: often, after booting up my machine after a day of not using it, it freezes on the booting phase, while showing the message "LOADING OPERATING SYSTEM" on a black screen, followed by a flashing _
After forcing the computer to shut down by holding down the power button, I try to restart it. But then it goes to a state where it powers on for a few seconds (not enough time to even show up the booting screen) and then shuts down, then restarts. This goes forever, unless I disconnect the power plug for a few seconds.
After disconnecting and replugging the machine to the wall, it powers up just fine, and this is me typing from this very computer right now.

Configs are as follows:
Gigabyte GA-Z68AP-D3 rev 2.0
Intel Core i5 2500k @4.2GHz (1.315v)
2 sticks of 4GB DDR3 1333MHz Markvision RAM
Nvidia GTX 760 Windforce 3x by Gigabyte
Corsair TX650M power supply
1TB Seagate 7200RPM HDD for OS and programs
500GB Seagate 7200RPM HDD for storage and downloads
Windows 10 Pro x64

What could be the main suspects?

 

[azaran]

First thing that comes to mind is a possible HD drive issue. Have you run any diagnostics on it? Seatools from Seagate would be my first step in checking out any hardware issues.
https://www.seagate.com/support/downloads/seatools/

 

[Daguin]

Thank you. I am downloading SeaTools right now and will give some feedback after testing both drives.

 

[Daguin]

I ran the long generic test on Seatools and both HDDs were approved.

 

[azaran]

Next thing I'd check, if possible is power supply. Do you have access to a known good PSU? Even a friends to test with would be a good start.