There is only one G.SKILL memory model listed in the 1333 speed section.
Did you ever try just using one memory module in the A1 slot alone? Did you ever set any BIOS settings for the memory? Did you ever check the memory settings, such as voltage (1.5V), or any of the timings?
There is a recent BIOS update, 1.60, that includes "Improve memory compatibility". Whether or not that would help, who knows?