I've titled this post like this because that's what I was looking for when this started to happen to me and as I had a lot of notes of what I was trying, I've recycled them here.
The monitor is out of signal. When running a game, even one with low requirements, after a few minutes (sometimes seconds) the monitor loses the video signal, it doesn't come back and we are left there. The computer continues to work normally, but with a black screen. You can even listen to the game, but you have to force a reboot or shut it down to be able to use the machine.
With very undemanding older games with low requirements such as Fallout 3 (2008) or Dishonored (2012) it works fine in low or medium quality for a little longer, but the error occurs again even when running in a window instead of full screen. Sometimes the screen goes blank, or another colour, for a few seconds before going black.
On restarting, everything continues to work normally, there is no heating, no abnormal fan noise from the graphics card or processor, and nothing to indicate that anything is being forced.
The art of troubleshooting is not to look for the right answers to what we think is our problem, but to ask the right questions to discover the real problem and above all to gather as much information as possible from different tests to follow a logical path of discards.
If you get a pimple on your toe and you look on the internet you might think it is gangrene and that the best thing to do is to have your leg amputated. I know this is an extreme comparison, but everyone I asked told me it was almost certainly an electrical problem and most of them pointed to the source.
The vast majority of texts that talked about very similar cases, even almost completely coinciding in the specifications of my computer, pointed to an electrical problem with the motherboard, the processor or almost certainly the power supply.
In principle, I ruled out a problem with the monitor because it is just over a year old. The graphics card was the candidate. Although I changed the thermal paste very recently, its fan works, maintains a good temperature and does not show scratches or artefacts on the screen, only when opening a game, which is one of the best tasks to test it. All other operations ran fine, even full screen videos and other heavy graphics tasks with editing programs.
Following different advice I did all these things before I found the solution. None of these tests resulted in anything other than the scary black screen of death. I have ordered them, more or less, from the easiest to the most laborious.
-Checking and cleaning all the power cables and connectors and all the fans. General cleaning of the inside of the case.
-Lower the monitor to 50Hz and check if you have the latest drivers installed.
-Limit CPU% usage to 90 and 80% and disable some services to optimise RAM consumption.
-Check for malware.
-Install and test games on SSD.
-Installation and testing of games on HHD disk.
-Cleaning of slots and connectors of RAM modules and changing the positions of the modules in the slots.
-Cleaning of the inside of the power supply and its fan, checking that it rotates. No excessive heat or noise.
-Delete and clean reinstallation of NVIDIA drivers for the graphics card with Display Driver Uninstaller (DDU).
-Test by plugging the GPU into another PCIe slot. Clean slots and graphics card connector and check for damage.
-Memtest64 test at max to check RAM, no errors.
-Windows Memory Diagnostic Tool, no errors detected either.
-I disabled Turbo Mode in the BIOS. No effect.
