1) Try using only one memory kit with XMP and see if it makes any difference. (Memtest and Prime don't always catch everything). In my experience putting two kits together like you have can be tricky to get 100% stable and is not a good idea if you don't know how to make the requisite adjustments.
2) Try setting PCIe to Gen 2 instead of Gen 3, in UEFI. Edit: ignore this as the card is gen 2 anyway.
-Raja
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