I had a set of 2x2GB G.Skill F3-12800CL9-2GBNQ running fine for around a year on default XMP settings, and just installed a new pair of these alongside them.
The system is now
i5 760k (no OC)
2 sets of the above mentioned for a total of 8GB RAM in 4 sticks
GA-P55-USB3L with latest BIOS (version F8)
To see if you have a 'bad' stick boot with (1) stick at a time, then test (1) Set at a time by itself.
If none of that helps then let me know. P55's are notorious for bent CPU pins and 'lost memory' is a typical issue. Also, if you have an aftermarket HSF then make sure there's no Metal -> MOBO contact.
Thanks for your post, jaquith
I've tried the one stick at a time approach, and kept getting rapid beeps, which the mobo manual said was bad RAM connection, and that I should try applying more force
I did exactly that, and the rig is working perfectly now
Apparently, the issue was that the new RAM was just not put in hard enough to begin with.