The most stable is 6.
If you want 7, I suggest you use Evga as your power supply.
What does manufacturer of PSU have to do with it? Nothing.
Just reduce amount of OC you have as when dual mining you are maxing out your GPU and it gives errors that crashed miner and system. Run all cards stock and slowly increase OC until you notice any crash/instability. Then go back a step and leave it there.