I have a ASUS Sabertooth X58, Core i7 950 CPU and 12 GB of DDR3 Corsair CMD12GX3M6A1600C8 memory and a Corsair 1200 power supply. CPUID reveals 12288 MB or RAM but Windows 7 (Professional) shows only 8 GB. I have the RAM set for 1.65 V in the Bios. This is a triple channel board.
This is how the cow ate the cabbage: I wiggled the CPU cooler a little to see if I could get it to make proper contact with the MB. BAD...the BIOS started to reboot three times before allowing the operating system to engage. SO I unscrewed the CPU cooler, removed the CPU, cleaned it up, reseated it, applied new thermal grease and reinstalled the cooler, this time, using some tiny washers I had obtained to better protect the back of the MO...missing in the kit. Fired it up and presto! All 12 GB memory now visible in the system.
Well, all sticks are good because I tested them all in the system and because they are all working now. The CPU was not making good contact with the motherboard. I think it could reoccur. My large, aftermarket CPU cooler places irregular pressure on the CPU socket. This may account for the error.