Gaming PC Crashes at Random - LFC

[Spoilers down below]
This modest gaming PC came in described as 'crashing at random', games would freeze or crash to desktop, etc. It's got some scruffy areas, so I cleared those up, but the real fault wasn't as easy to resolve.
Had memory errors with XMP turned on. RAM kit not on the mobo's QVL, and not faulty. Opted to downclock RAM from 3200 to 3000. Custom RAM timings will also have worked, but I don't really want to do custom timings on a customer PC.
If you want to know more about solving this issue with custom timings, check out this video: • LBC#43 - Setting Up XM...