I ran into a very similar problem wanting to max out my RAM. My mobo has limitations do to the cpu when using RAM at 1066 mhz populating all the slots. I was BSODing like crazy. I has to do with the RAM controller in direct relation to the CPU. I can use maximum RAM at that speed (1066mhz) for 8 gigs but in only two slots, not all 4 slots...looks like you have something similar my friend. You may want to contact Asus support staff for verification and a course of action.