Hi newcomer and welcome to the Tom's hardware forum.
What mobo do u have? Do this:
Run msconfig, boot tab, advance options and check the part that says "Max memory" shouldn't be selected.
Did all that. It only happens when I overclock. Looking at some forums, it's apparently a widespread problem. At default settings, everything is fine. When I overclock, even a very small overclock, 2 gigs of memory disappear.
I'm on an EVGA x58 sli motherboard.