Question Computer keeps rebooting at startup

Nov 5, 2019
Completely out of the blue, last Sunday I realized that my 2011 custom built PC can't power up. I hadn't turned on the computer for about 2-3 weeks. The problem is the following: after I turn on the computer via the power-on case button, everything seems to start working and after ~3 seconds, it all shuts downs and then after again 1-2 seconds, it turns on by itself, then shuts down again, and so on it enters this notorious loop. I see nothing on the monitor, we can't even talk about reaching the BIOS.

My investigations:
  • if I remove the power cables from the PSU to the video card, the PC seems to boot successfully and does not enter the boot loop, but I have no display...
  • if I remove the video card altogether from the motherboard, the PC can't boot and enters the same boot loop I described in the above paragraph
  • I tried disconnecting from the PSU all of the other components (except the CPU), and booting the PC, but it didn't help. In a nutshell, 0The only way I can avoid the boot loop is by disconnecting the video card from the power supply (as I already mentioned in the first point here).
What I have done (with no success):
  • removed the CMOS battery and installed it back after ~10 minutes
  • tried to clear the BIOS settings via the jumper method, using the CLRTC pins - I did by moving the jumper, then turning on the PC (as I followed an ASUS guide)
I have not tried replacing the CMOS battery with a new one!

The system:

CPU - i5 2500k;
MOBO - ASUS P8P67-M PRO B revision;
RAM - 2x4GB or 2x8GB (wow, I don't remember) Kingston HyperX Predator;
PSU - Corsair CX600 600W;
HDD - Samsung Spinpoint F3 500GB;
OS - Windows 7 Ultimate 64bit

Any ideas? THANK YOU!