amavisd fails to start up

I've got something squirrelly with my xserve running 10.4.6. It seems almost anytime I restart the mail server, amavisd doesn't restart correctly. Mail backs up in the queue for a couple hours until someone notices and then starts complaining far and loud. Typically, to fix this I go into /System/Library/LaunchDaemons/org.amavis.amavisd.plist and change the Disabled key from true to false and kick launchd to make it reload. For whatever reason, that change doesn't stick. The next time it happens (weeks or months later), the file has magically changed back. So what's with that? Is amavis supposed to be launched another way? This is pretty much a vanilla installation with as little customizing as I can get away with. I've seen references to clamav and amavis, is clamav supposed the launch amavis instead of launchd? I have the feeling I'm fixing it in the wrong place, since it happens over and over. thanks, jay