I've got my reference sapphire at 820Mhz with 1.44vcore 24/7 for several months without a problem. I have tried up to 1.6 vcore for benches without a problem. I couldn't get it 100% stable at 840Mhz no matter what voltage i used, even at 1.6 it wasn't 100% stable. The only problem that i have found is that at high voltage during furmark, when i'm not stable (for example when i was trying for 840) and i got artifacts, these artifacts wouldn't leave until i shut down my computer. After that everything was normal. With just a reboot the issue wasn't resolving and i don't know why.