Question RX 480 won't boot ?

Mar 22, 2021

I just built my new PC

CPU: i7-10700k,
MB: Asrock z490 Steel Legend
RAM: HyperX DDR4 3200 (2x16 GB)
PSU: Chieftec 750

The XFX RX480 is from the previous build which was working fine.

The problem appears since the first run - VGA red indicator lights and PC won't boot but, after a restart or off/on power - it starts work and everything is fine until next power off the PC for more than a couple of hours. Then problem appears again, but an odd thing - if I turn off my PC for like 5 min and then turn it on again - everything fine! I've tried reinstall GPU a few times, update to the latest Radeon drivers, update BIOS, reset CMOS, remove the battery from MB, use different display ports and HDMI cables. But the problem still there. Sounds like something obvious missed, but I can not find it. In BIOS - primary GPU - PCI express and integrated GPU is disabled.
Can anyone help?

P.S. Sorry for my English, but I really can not find the solutions.

Continue to investigate the problem.
Found a couple of things:
1) My old PC was built on MB: asus p8h61-m evo which does not support PCI-Express 3.0, which means previously my GPU was working in PCI-E 2.0 slot.
Question: Could it be a reason for my problem? I've found a couple of topics about rx 480 and PCI-E 3.0 slot problems. Now I've put my PCI-E 3.0 slot in compatible to Gen2 mode in Bios. (it works, but did not try to shutdown it for more than 20 min)

2). In Boot section of Bios I noticed that in CSM part was something like this:
CSM = Enabled
Launch PXE OpROM Policy = Legacy only
Launch Storage ROM POlicy = Legacy only
Other PCI Device ROM Policy = UEFI Only.
I'm not sure about all these settings, but now I turned CSM to disabled at all. What are the correct settings for this? (It works,but also did not have a chance to shutdown for a long time to check)
Also, could it be a reason for the problem?
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