Did this ever get resolved? I have the exact same problem with a very similar pc setup. The only difference is the psu and the memory is slightly different (6GB 8-8-8-24 CMP6GX3M3A1600C8). Everything that is described above is happening in the same way, standard motherboard config detects 6GB every time, but when overclocking from cold start only 4GB is detected.
Any help would be greatly appreciated as I have gone through so many different threads and configured the bios in different ways. This is the first thread that i have come across with almost the same config, and describing the cold boot.