Mains power is somewhat unreliable where I live, so every now and then, the computer gets rebooted when the PSU can't maintain proper voltage during a brown-out or momentary black-out. It's happened a few times recently that when power is restored, the BIOS POST completes successfully, GRUB starts to load and then freezes. I've seen this at the Welcome to GRUB! message, but it seems to happen more often just past the switch to the graphical OS list. At this point, the computer will not respond to anything (arrow keys, control commands, Ctrl+Alt+Del, ...) - it simply sits there displaying this image, seemingly doing nothing more. At that point, turning the computer off using the power button and letting it sit for a while (cooling down?) has allowed it to boot successfully. Turning the computer off and immediately back on seems to give the same result (successful POST then freeze in GRUB). This behavior began recently, although does not seem to be directly correlated with my hard disk woes (although it may be relevant that GRUB resides on that physical disk, I don't know). Once the computer has booted, it runs without a hitch.

I know that a "proper" solution would be to invest in a UPS, but what might be causing behavior like this? I was thinking in terms of perhaps the CPU shutting down as a thermal control measure, but if that was the cause then wouldn't I see similar freezes during use (which I do not)? What else could cause freezes apparently closely but not perfectly related to the BIOS handover from POST to OS bootloader? The BIOS settings are to reset to previous power status after a power loss. Since the PC in question is almost always turned on, this means restore to full power status. I have no expansion cards installed that make any BIOS extensions known by screen output during the boot process, at least, but I do have a few expansion cards installed. Haven't made any changes in that regard in a long time, now.

I haven't touched GRUB itself for a long time, whether configuration or binaries, so I don't think that's the problem. Also, it doesn't really make sense that a bug in GRUB would manifest itself only once in a blue moon but significantly more often after a power failure.