Linksys WAG200G-EU stops routing UDP after a while (scripted reboot how-to)

It seems that after having been up for a couple of days, the WAG200G starts having issues routing UDP packets properly. This particularly affects VoIP traffic (here IAX2 on port 4569). The symptoms are that "regular surfing" works flawlessly, but the registration with the asterisk server fails. tcpdump shows no traffic on the server side. Restarting the WAG200G immediately solves the problem.

Here is how I restarted the router from the command line:

wget --http-user=admin --http-password=pa55w0rd \
     --post-data='reboot=1&save=Enregistrer+les+param%E8tres&todo=reboot&h_reboot=1&this_file=Reboot.htm&next_file=index.htm&message=' \
     http://192.168.0.1/setup.cgi

(replace password and IP address as appropriate)