This is sort of a long shot, but its something thats recently worked well for me. My system is very different from yours, but its also custom built personally. Anyway, turns out many of my performance issues in many games were attributed to an insufficient PSU. I had a single 12v cable at 15A running all of my drives, dual processors and my graphics card. Recommended nowadays is 2 12v cables over 18A and having one running the graphics card and the other running the rest. Now, newer systems then my own would probably require a higher ampere rating on the 12v cables in order to run the higher powered equipment.
Now you may have this covered, but it was something I dealt with recently, so I figured I'd add something new to this thread. Hopefully it helps you or someone somewhere. From looking at your specs, I'd say you need alot of power, so I'd double check that to make sure your ratings are up to par. Also try a voltage regulator. The one I have is wonderful. It guarantees my computer gets a nice clean 110vs. I've noticed a huge boost in performance of all of my recent games with my new PSU and AVR components.
Good luck, I wish you all the best in your pursuit to getting RoL to run properly.
PS. I still cant get bik movies to run on here. Again, something to do with windows not liking my AMD system timer. Whats really odd is that bik movies run great in some games and horrible in others. Oblivions movies run fine, RoL's movies run awefully. Oh well, such is life.