Intermittent losses of connection

Hello,

I have Windows 2000 Advanced Server that continuously loses the
connection to Internet during short spaces of time.

I have prepared screenshots that help to understand what I mean:

http://www.sunglowal.com/capturas

I have verified network interfaces, TCP/IP configuration... and
everything seems to be correct.

As can be observed in one of the screenshots, the connection appears
active for more than 3 days, reason why I think the problem could be
located in some other element of the network, like cables, router,
firewall...

I will thank very much any comment or suggestion that can help me to
solve the problem.

Greetings.