Question Sudden Memory Problem, plz help


Sep 19, 2013
I'm running a home built system;
Asrock z97 pro 3 mobo, i5-4670k cpu, Geforce 1080ti gpu, Win 10 pro 64bit os, with four sticks of 4gb corsair dominator-gt ddr3 2000mhz dram.
I know it's cpu bottlenecked and a Frankenstein of a system, but it's been working fine for me for quite some time now, playing intensive, modern games on ultra settings as my monitor is limited to 1080p.
Just recently, I started getting crashes in games and upon investigating, I discovered that my normally xmp profiled dram at 2000mhz was set to 1066mhz.
When I reset to normal xmp 2000, it keeps crashing and by experimenting I've discovered it will run fine at 1866mhz but won't go higher.
btw, I have six 4gb corsair sticks, so have been able to systematically swap out each stick in turn to see if it's one stick gone kaput, but that hasn't alleviated the problem.
Also I ran the Windows memory test and it reported all as fine.
Could it be a mobo problem?
All helpful suggestions for thorough diagnosis and/or potential fixes will be appreciated.