Internet sharing turns itself off?

Hey guys... My mom has a Mac Mini running Tiger, 10.5. something, very likely the latest, as she faithfully updates. There are a couple of Windows laptops in the house as well that share her broadband connection from the Mini via internet sharing. The problem is that Internet Sharing, as found in System Preferences, seems to randomly turn itself off, which is frustrating for my dad, who doesn't know a thing about Macs and is sometimes left trying to get connected while my mom isn't home. I'm always their 'tech support', and I have no problem telling dad how to turn internet sharing back on over the phone, but I'm wondering why it turns itself off in the first place. The lock has been locked to keep changes locked in, so I'm not fully understanding why it turns off. Seems it happens if it is restarted after updates and also happens even if the Mini has been left on for days or weeks.

I googled the problem, but all I found were a bunch of suggestions that really weren't problem solvers. Any insight? Thanks!