Troubleshooting hardware problems

Memory (RAM)

• The Gentoo packages memtest86 and memtest86+ provide tests for the memory in your computer. However, there is no way to run these programs from a running system. They must be ran from a special bootloader: the Gentoo packages basically install that bootloader and add an option to GRUB to start the tests.

GPUs

• If you use a riser cable to connect a GPU to your motherboard PCI Express bus, be extremely careful that the cable should be certified for a high enough PCIe specification / generation. I had an important issue when trying to install a MSI nVidia GeForce RTX 5080 in a Meshlicious case. The card was connected via a riser cable to an AsRock Phantom Gaming B850I motherboard, which supports PCIe 5, so by default the PCIe bus runs at this specification, and the card supports PCIe 5 as well. However the cable could not cope (it was good enough for PCIe 3, maybe 4) with it and it created very strange issues (extremely slow display during boot on the BIOS via a DisplayPort cable, impossible to boot into Windows, the nVidia drivers would not start the card under Linux...). Forcing PCIe 3 in the motherboard BIOS immediately fixed all of the issues, and a better riser cable would probably have worked as well.

Nvidia GPUs

• I had a case where an nvidia card HDMI output would no longer work (black monitor), even during power-up and bringing up UEFI / BIOS interface. Booting into Windows (using the built-in Intel HD HDMI output from the motherboard) and then switching the HDMI cable to the nvidia card "resurrected" the card. Windows output was even working instantly without a reboot.
• Make sure you have a powerful enough PSU (Power Supply Unit). I had a case where the computer would freeze very often when using a 600W PSU (and a nVidia GeForce RTX 3080 Ti). See this report. Switching to a 1000W PSU fixed all the crashes.

EVGA nVidia GeForce RTX 3080 Ti

• The card worked, but produced a blank screen at boot (during POST/UEFI BIOS, GRUB bootloader screen, and Linux console driven by the framebuffer driver). The screen only started after Linux X11 (or Windows) has booted. This made it very difficult to access the BIOS or the GRUB dual bootloader.
• Note that this issue usually happened only when a cold boot was issued (or a reboot triggered by the BIOS/GRUB). Just after a reboot from Linux or Windows, it worked and I could see the BIOS screen.
• If you're unable to get a BIOS screen, it's very dangerous to keep the "Wait For F1 If Error" setting enabled on the BIOS configuration. Any error during the boot process won't be visible, and will prevent the startup of the system as a result. It will never reach the stages of X11 or Windows loading, which would power up the display at last.
• To fix this situation, I tried to update motherboard BIOS, GPU VBIOS and firmware without any result.
• Update: this issue was at last fixed with a firmware update on the monitor (display, which was an Asus ROG Swift PG32UQ model). So apparently, the issue was linked to the monitor, and it was fixed by the firmware update.