What's the Kombustor?
Kombustor is a little GPU stress test that you can launch from Afterburner. It's similar to Furmark I think. It's supposed to give you a way to see if your overclock is stable.
Isn't EVGA Precision the best tool for GTX 670/680 cards? I'd recommend that from what I've seen - should work fine even if your card isn't an EVGA.