I run just about the exact same setup as far as core components, except I run 2 840 Pro SATA SSDs in RAID0. I have my background clients set to 12fps/renderscale 0.3 and I have zero issues maintaining 60-90fps out questing. It dips in Orgrimmar around the AH on saturday evenings but otherwise is stable. I'd suspect something like a bad driver or some other option that got toggled on unexpectedly. Do you have a "detail" macro that does things like toggling shadows/etc. when swapping a client out to the background?