Yes try increasing VTT as well as CPU voltage slightly.
Mixed pairs? could be the issue.
I thought Sandy Bridge had a 1.5v limit on RAM?
Installed without a hitch running at 2400. I got this as I wanted 16GB, but I have to say it looks marvelous in my Enthro luxe with the case lights on
Mine refuse to run at 2400 even with no overclock and just using the XMP! 2133 seems OK at the moment!
Not all Sandybridge will do 2400Mhz IIRC.
To be honest, It's not likely to make a difference anyhow!