I have an EVGA 780i and I found this approach to work well for me.

1) Adjust your vtt to a low enough voltage so that prime 95 will fail after one or two iterations. ( I had mine around 1.5 volts @4.2 GHz.)

2) A good starting point for me was .30 on all four lanes.

3) boot up and start prime 95 on all for cores. and see which cores fail first.

4) Gradually raise or lower the voltage for the particular core that is failing first. (this can be a time consuming thing because it really is trial and error.) I found that core 2 would fail first and than core 1. core 0 and 3 were pretty stable until I got too high on the gtlref voltages. The funny thing was that it seemed core 1 needed more voltage than the rest and core 2 needed less. After some time I got all 4 cores to go about 3 iterations before failing. ( I ended up at .40, .50, .25, .40.)

5) now go in and raise your vtt up around .25 volts higher. (1.525 in my case) my system is now much!!! more stable than it was when I had them all on auto.
I hope this is helpful for someone because I know I was at a loss with a system that was un-stable for some time until I figured this out. GOOD LUCK!