How high of a octane are u runningIt's possible, premium has a slower burn rate than a lower octane. Running a higher octane than the engine needs is flushing money down the drain. Make the dealer install spec plugs - they're idiots for that reason alone.
fwiw: since the 700 seems to have one plug that burns blacker than the other, I'd try a one step hotter plug in that location. This is because I'm a closet motorhead, so proceed at your own risk.