I’m now running with intel_pstate=disable on kernel args and haven’t had the problem since (looks like it still happened when setting everything to performance or with just one core on powersave, just way less often), so the problem kind of lost priority for me given the lack of interest.
Playing multiple videos really is a reliable reproducer for me (with default powersaving options); so given some said they can’t get it to crash and more importantly given that almost no-one else reported the problem (we’re, what, 5 so far? For a problem that happens multiple times a day with regular computer usage anyway!), I’m thinking this is some subtle hardware problem in a bad series of chips and call it a day; if more folks show up here it might revive some interest though, curiosity stll wants me to probe the cpu voltage when frequency varies and things like that, just not enough time to do everything.