the z270i STRIX board has issues with cold boots when your CPU is overclocked. I dont know if that applies to overclocked RAM though. If I were you id just set the RAM to the default XMP its rated at. My 7700k is stock at 4.5GHz and my DDR-4 4000 kit is stock and the 4000MHz XMP, and i have no cold boot issues. If I OC my CPU though, then I will have cold-boot issues. I think the cold-boot issues were related to not enough voltage being sent when your voltages are farther from stock, i dont remember exactly. just google "z270i strix cold-boot issue", you might find something.
Also make sure you update your z270i BIOS. Hope this helps.
Cheers for that, but i am actually running the ram at default XMP timings and its still madly unstable, but will boot from time to time. I am wondering if i should just try to set the XMP manually, to see if that is better than the auto setting.
Also i am running the ram together wih a i5 7600K (not oc'ed) and a MSI AERO 1070 (not oc'ed).