XMP used to be stable, but stopped being stable

[Pinossar]

This has happened to me a couple weeks ago, nothing changed, I got the RAM a couple months ago, and was immediately able to run it at XMP settings (3200MHz CL16) and was eventually able to push it to 3800MHz CL17, but it suddenly tried to reset the bios, due to a failed OC, I dismissed it, and reset the PC, to it not POSTing, and after resetting again, reset the settings again, I disabled both the cpu and ram oc, booted normal, and then after messing with the settings a bit, found out it was XMP that caused it, no ammount of changes to the BIOS allowed it to be enabled, without either the pc not POSTing, or crashing Windows while it was loading. The most I can now push the RAM is 2800MHz CL16. Could this be an issue with the RAM itself, the CPU's memory controller, or the motherboard itself? Would it be possible to diagnose and possible fix the source of the issue?