My current system (ASUS P67 Sabertooth, Intel i5 2500k, Radeon HD 6950, 8GB (2x4) Corsair Vengeance RAM) is stable. I am trying to add two more DIMMs of Corsair Vengeance RAM (the exact same brand and model as my current RAM) to bring it up to a total of 16GB RAM. According to the mobo manual, the mobo supports 4 DIMMs. The problem is when I add the RAM the computer won't boot - the DRAM LED just stays lit. I've tried using the motherboard's built in "MemOK!" feature to no avail.
Try swapping the modules around in the dimm sockets even if it is the same modules, clear your cmos (refer to motherboards manual) if that does not work take the memory back and swap it for a 2x8 gig memory kit and sell those you already have.
You could also update your motherboards bios to the latest version if you want to stick with this memory but don't update with a beta version. The motherboard could have a issue with assigning timings for these modules.