I've experienced random locks (pretty fast on heavy prime95 test) until I downed my memory from 1600 to 1333. Everything is just fine after that.
No matter what I've tried, I was unable to run this motherboard with 1600Mhz RAM (KHX1600C9D3K2/8GX = Kingston 2x4 kit, 9-9-9-27)
Manually set timings, tried 1.64 -> 1.7 voltages, T2 etc etc. No luck. But it's rock solid @ 1333.
ps. tried F7E BIOS is well, different dimm sockets to be used - nothing helped.