Have you tried different brands of pads? Try changing only one thing at a time until you notice a difference in the problem. At that point you will have come close to isolating the cause. But from your description, it would seem that it is a pad issue more than a rotor issue. Does your brake pedal "pulse" or "shudder" when you are braking? That would probably indicate a rotor issue. "Grinding" as I would describe it would be the sound of worn out pads contacting the rotor surface... A metal on metal sound. If it doesn't grind all the time while using the brakes, I don't see how it would only grind occasionally. Possibly you are describing something else? Try bleeding and flushing your brakes also?
