LlC should definitely not have anything at all to do with your problem. That's just for stabilizing/tuning cpu and cpu-nb voltage when overclocking. Pull two sticks out for a while and run it at 1600. If it runs stable for a while, it's just the IMC you need to work on.
That's the easiest way to either confirm or eliminate filling up all four dimms as a potential trouble spot. In some cases, you just pay a penalty for running four sticks; either speed or timings. If it makes a huge performance hit (doubtful), you can always get a 2x4G set instead.