Note that keeping both sticks in the A slots (or both in the B slots, for that matter) results in single-channel mode. That is likely why they are working, as single-channel mode is less stressful on the CPU's memory controller.
What's the G.Skill part number for your RAM kit?
Things that help memory stability:
RAM sticks with Samsung B memory chips
Using 0902 BIOS (
get it here - elmor is ASUS R&D and a world-renowned overclocker)
SOC adjusted higher (1.2v is considered maximum for 24/7 usage)
VTTDDR (first setting in Tweaker's Paradise) adjusted higher (0.80520 or the next step up)
DRAM Boot (bottom setting in External Digi+ menu) adjusted higher when using 1.35v for DRAM voltage (Auto is 1.2v and it needs to be at least 1.35v for 1.35v RAM to boot correctly)
Read through that Overclock.Net thread for even more advice.