Just throwing another suggestion to look for, maybe check your RAM usage? When I was first trying Linux i was using a persistent live iso, and it would randomly lock up like you describe. Just about the only functionality I had was to REISUB lol. It turned out my OOM services weren’t closing programs until it was too late … or maybe it was trying to put memory into swap space that didn’t exist. Who knows. Try opening firefox a bunch of times and maybe see what happens
This type of thing shouldn’t happen with a normal setup though.