I am always running on stock core and voltages.
One time I tried raising the voltage to around ~1.063. Core clock to ~832. Cant really remember exactly. Ran pretty stable on other games.
But on BF3, still crashes. However, when I check the Asus GPU Tweak Monitor, I noticed that max voltage only reached 1.025.
Maybe my clock speed is not enough?
Did you try running the card with 1.025V core voltage and 772MHz default core clock?