My modem/router doesn't like Ubuntu

[Óscar Fuentes]

My internet access consists on a Xavi 7768 ADSL modem/router. It is
connected by wire to two windows machines and a Kubuntu 8.10. After some
hours or days, internet access on the Kubuntu machine becomes flaky,
most connection attempts takes a long time or timeouts. The Windows
machines have no problem. The Kubuntu machine can connect to the other
machines on the local network at full speed, though, so the problem is
limited to connections to/from internet.

Tried disabling ipv6 on Kubuntu, no effect.

It is not a DNS problem: pinging by IP address with the -n option is
slow too. The most strange thing is that ping seems to hang for several
seconds: it doesn't react to Control-C immediately. The reported times
(when ping obtains a response) are on the range of milleconds, when ping
takes several seconds (typically 10 or more) to report responses. With
`dig' I see something similar: it seems to hang for several seconds and
then either reports a timeout or says that the response took a few
milliseconds.

The only cure for this is to restart the router. Then internet works for
the Kubuntu machine for some hours or a few days until the problem
appears again.

If this is a problem with the modem/router, is there some setting I
could try to avoid it?

-- 
Oscar