Thanks for all replies, in my case the issue turned out to be EasyTune 6, every time it ran it dropped my memory from 1866 back to 1600MHz in the BIOS, no matter what settings I tried using in it. Once I disabled ET6, memory stays at 1866MHz with no issues, apparently some kind of bug in the software I ran across.
I like ET6 because it keeps my fans running quietly, yet still allows adequate cooling, hopefully my issue will be resolved in a future release.