I'm kinda astonished that nobody mentionend a bios update before. (neither me
)
had some trouble to get my 1866 RAM working on full speed but after the bios update I found an extra profile with the correct RAM-timings and since then it works like a charme...
edit: ok, just read you stated to already have the latest bios. in this case looking a bit around in that bios can't hurt. I found that profile in the bios of mine under d.o.c.p. (dram overclock profiles). auto-detect and memOK! didn't work and always set it back to just 1333...