I just bought 2 kits of Patriot Viper II Sector 7 6gb (3 x 2gb). If I boot without any overclocking everything shows up correctly with 12gb. However, if I overclock the CPU at all, with the ram just a tad above 1600mhz (which is its advertised speed), the ram will still show up correctly at first, but after rebooting, it shows up as only 6gb. I've tried adjusting the core voltage as well as the dram voltage, but it still only works temporarily.
Here is my system:
Asus Rampage III Formula
Intel Core i7 920
2x Patriot Viper II Sector 7 6gb
I've fixed the problem. Thanks everyone for their help. While I did experiment with the QPI/DRAM values that didn't really work. Asus included a memory recheck feature in one of their bios updates that I enabled. This seems to have solved the problem