Network connection stops working after 3 days

Question

My Mini is never switched off, but is often put to sleep. After near-faultless operation since new, it has recently (possibly coinciding with the upgrade to 10.4.10) started having a recurring networking issue.
After a small number of days, the network connection becomes unusable - neither Safari or Mail will work. Strangely other tools appear OK (e.g. traceroute www.whereiwork.com worked fine). Rebooting always temporarily fixes the issue.
The time interval may vary, but on this most recent occassion I checked the uptime before rebooting, and it was just 3 days.
Next time it occurs, what can I do to identify the cause of the problem?

Answer 1

Well, just under 4 days since the last reboot and it's done the same again...
It seems it's not entirely failed, but certainly doesn't work well enough to be useful.
e.g. if I start a ChickenOfTheVNC session to another machine on the LAN it appears to make a successful connection and creates a window of the right size but never fills it in. The server machine agrees that a connection has been made, but disconnects the client after a long-ish timeout.
Some other semi-random things I've tried:
/etc/resolv.conf has an entry in it referring to my router.
ifconfig en0 down
ifconfig en0 up
...makes no difference
Unplugging, then replugging the Cat5 makes no difference.
Rebooting the router makes no difference.
Not knowing enough about the inner workings of OS X, I'm going to have to resort to a machine reboot again. If anybody has any bright ideas for debugging this, it's due to go again sometime on Saturday afternoon.
(The procedure for "rolling back to .9" seems more disruptive than the problem itself, for now at least...)