Northbridge overclocking brings you up to nowhere. Trust me it's not worthing the instability.
Be glad that you even reached 3.9ghz mine is reaching only 3.6ghz in modest Vcore voltage.
If you want to risk it though than you have to go (if you have an Ami Bios) under 2. or third tab and raise the Mhz in small 5mhz steps until you've reached your Destination.
Take on the FSB and blow this up until you've reached your goal.
Now happy overclocking fellas