Mac Pro 2013 6.1 (Screen goes black in exactly 60 secs but still run)

Hello everyone¡ I have a Mac Pro Late 2013 6.1 that screen goes black after exactly 60 seconds. Does not matter if you're on Windows, Mac OS or stay on Boot picker. It takes exactly 60 seconds for the display to go black (But it's still running since you can click on a key and will sound or youtube keeps playing audio) so it doesn't go to sleep just black display, but if I unplug and plug HDMI again it works for another 60 seconds.

I did reset PRAM and NVRAM as SMC as well, I took it completely apart, cleaned all dust and replaced thermal paste with no changes.

Also tried to disconnect one of the GPU's (The one without hard drive slot) and Boot with just one, and did the same but lagged.

Tried to Reflow the main GPU (With a professional automatic rework station) and still no changes.

Also replaced the RAM memory and placed new ones in different slots with no changes either.

Also performed a diagnostics and All seemed good.

Any ideas? ( I think that maybe VideoRam bank on the GPU might be defective and when the moment of reaching that bank comes it goes black) since the GPU works for 60 secs with no issues... THANKS FOR YOUR HELP