Yeah, Memtestx86 is extremely thorough but it's also not really a stress test, it's more of a test for faulty silicon. Timing faults aren't always caught out by it so as
@freeagent says; TM5 on anta777 profile.
If it's passing anta777 and you're getting very infrequent crashes, you should maybe check your mains power delivery. Old power strips or failing surge-protectors can do weird things that cause clock instability in otherwise stable systems. Perhaps you have something noisy plugged in on the same circuit like a fan or motorised gadget, or perhaps you have some arcing in that circuit that's causing occasional brownouts to exceed your PSU's hold-up time.
I dunno man, you've actually done most of the obvious stuff and I've been doing this profesionally for 20+ years so my "obvious" list is pretty damn comprehensive. Confirm it's BIOS defaults and 2133 and run TM5 anta777. Do you by any chance have another RAM kit to run for a few days?