On stock multiplier with auto voltage, it is usually overvolting enough that vdroop should not cause instability. Few of the enthusiasts here are electrical/computer engineers, and few have experience with 1.35v DRAM, so you should not be asking and expecting good answers about that. How about buying a 1.5v kit and switching it alone to maybe isolate the problem and bring more knowledge to these forums?
IBT and Prime95 have always been able to reproduce system stability issues for me, so I hope you can prove that it is or is not graphics related through those tests. If you can get instability outside of games, I suspect that your memory configuration may need some work. My X58 system wasn't stable at stock, and it turned out that I had to goof with the memory configuration a bit to make it stable. Maybe it was because I use 1.5v when 1.65v was more common for that platform.