100% CPU long-running children

[E.B.]

Hi,

I recently found two amavis child processes on one of our mail servers
using 100% of one CPU and running for a very long time. Some questions:

1. Why would this happen?
2. Isn't there a safeguard against this?
3. What's best way to handle this situation?
4. How to best monitor this in the future?
4a. General (non-amavis-specific) server monitor (nagios?) would probably
have helped identify this earlier. Suggestions for that are invited

I left the processes running for now in case there is anything I can do
with them to get useful information about them. In ps, they look like
this:

 2792 ?        R    70151:33 amavisd (ch12-02792-12)
11348 ?        R    14120:41 amavisd (ch20-11348-20)