IS the 8pack stuff that temperamental that it wont work at XMP?
They shouldn't be. When you said you ran memtests did you run say testmem5 for a couple hours atleast? If you only ran memtest86 (not recommended btw) then more tests are needed to see if you just got a bad kit. If you need it: https://www.overclock.net/threads/memory-testing-with-testmem5-tm5-with-custom-configs.1751608/