Recently my desktop PC suddenly rebooted itself after 10 minutes of usage. While it was booting up (Ubuntu 20.04 boot screen visible) it rebooted itself, and that went on forever. It never reached the login screen.
I started memtest86+ from the grub2 boot and let it check the RAM. With all 4 RAM sticks inserted, it hang at 30% on test
Test #2: [Address test, own address Parallel] Testing 4096M - 6144M
The left top of the screen showed "memtest86+", with the "+" in red and blinking. It continued to blink, but the test progress was stuck at 30%.
For each RAM module, I removed it and ran the tests again. Test #2 stuck at 40% then.
In the end, the RAM modules were all fine, and memtest86+ 5.01-3.1ubuntu2.1 shipped with Ubuntu 20.04 had a bug. The tests ran fine when running a memtest86 version I downloaded from the official website and booted it from a USB flash drive.
I was irritated that the "+" on the screen was still blinking, although the test hang. It seems that the ANSI blinking is executed by the graphics card itself, and so it continues to blink even though the underlying program hangs.
The culprit for the boot problem was my graphics card (MSI R7 370 with 4096 MB RAM), which broke after 7 years of usage. After removing it and using the integrated graphics, the PC booted without problems.