Prime95 was still going when I got home. 191x21 works with 1.4 V Vcore and 1.375 V QPI. 200x21 will not work with the same settings, or with QPI at 1.4 V. I'm hoping to keep the vcore under 1.4 V. Any suggestions?
Edit: Could this be because it's trying to run my ram overclocked (barely)...
Turbo mode has been disabled.
(191x21)
Vcore 1.4 V
QPI 1.375 V
I'll try for a higher speed and lower voltages when I get home. Prime95 was stable with these when I left for class this morning, we'll see when I get back.