I didn't realize the difference that Gear Down Mode makes for stability (I'm very new to RAM overclocking). I did 2 hours of Memtest5 without any errors on safe timings, so I'm calling this a milestone for stability. I'm still going to tighten tRC to 77 and tRAS to 40 and run 4-6 hours on MemTest5 and 2 or more hours on ycruncher. (VDD and VDDIO is set to 1.470V.)