N360 continuous disk writes

About once per month, N360 executes a near continuous disk write that slows my computer to a crawl.  It causes a very slow Outlook startup (on the order of 10 minutes) while the Norton Anti-Spam plug-in loads.  The continuous disk write goes on for a couple of days.  I can't tell what file it is writing, but it is at a high data rate on the order of 2MB/sec.

I'm running Windows 7.

I previously ran scans for malware, and have found nothing.  I can't figure out what files are being written.

How do I make this stop?