I've always been a fan of memtest86. Passmark (ew) bought em - but there's an open sourced version:
https://www.memtest.org/
Make sure your sinks/fans are clean and loop this overnight. Also, most shops will plug a tester into your power supply for free. Sometimes weird stuff can happen if the caps wear and voltages aren't exact.
Two simple and (relatively) cheap fixes. Good luck!
-Mcoy