Have you run MemTest on each stick yet? I had a very similar issue on my Rampage II Extreme where it reported 2Gb or 4Gb...sometimes 6GB after a hard reboot. I thought it was the board until I tried them in a different mobo. Ran MemTest and sure enough, one of my Dominator sticks were showing tons of errors.
Intel i7 3930K @ 4.2 Ghz | Asus Rampage IV Extreme | Asus GTX TITAN SLI | 16GB G.Skill Ripjaws Z DDR3-1866 | Corsair H80 | Corsair Force GT 90 GB SSD | 2x Corsair Force GT 240 GB SSD | 450GB VelociRaptor | SB X-Fi | Corsair AX1200i PSU | Logitech G500 | Logitech G110| CM HAF-932 Black Edition | Asus VS248H-P | Win7 Ultimate 64-bit
Lenovo Z575 - A6-3400 | 8GB Kingston HyperX DDR3-1600 | Kingston HyperX 240GB SSD