I have a laptop running Arch that I've been using for about 3 years now. Admittedly, I'm a bit sloppy on system updates, but it's in great shape aside from a few quirks with the graphics driver. This changed overnight yesterday.
Somehow, the laptop shuts down spontaneously in very short intervals, to the point where it's extremely hard to even diagnose what's going on (typically, after startup, I have 1-2 minutes to do something until it decides to shut down). The shutdown is "orderly" in the sense that it doesn't just turn black, but instead I see the shutdown log. Upon next boot, I see some checks being run because the file system was not unmounted properly, but other than that, I see no indication of anything being wrong - very similar to when the laptop shuts down because of critically low battery, only that here, it happens irrespective of whether it's on battery power or on AC.
I've asked friend google, and it seems that most of the time something like this happens, it's related to overheating. Hence, I've looked at the temperature sensors and found that immediately before the shutdown, none of the temperatures was even above 50C, which makes it seem very unlikely to me that this is the issue.
I'm at a loss - what are possible causes for this behavior, and how can I fix it? There was nothing specifically I did with the system before it started, no full system update, installation of new hardware or physical accident. The most recent to changes I can remember performing was updating a few userspace apps (discord, skype, chrome) and swapping between the two batteries I have for this laptop a few times, but none of this should in any way be related to this behavior.
Usually, I would now start reading up on the internet, but since most of the search results are related to overheating (which doesn't seem to be the problem in my case) and I don't really have enough time with the machine running to sensibly debug and test hypotheses of my own, I'm coming here for guidance and suggestions on what to check.
Any ideas, suggestions and checks to run are highly appreciated.