I just purchased a refurbished nMP, late 2013, 6 core with dual D700. on latest Yosemite release. Issue: regardless of display connected, I get bad glitches, quick vertical shifts in my GUI output. Monitors tried: NEC PA272W, Apple iMac (TDM), Eizo CG247. Cables tried: Thunderbolt to Thunderbolt, Thunderbolt to MDP, Thunderbolt to DP. Can't be the monitors. Running "native resolution" (2560x1440). Bad MacPro? Bad OS?