Internet connection dropping

What would cause a computer to stop connecting to the internet after about 5 mins?

The computer is running xp pro and is on a DSL connection and has plenty of RAM

you can browse the internet for about 5 mins then no matter where you go you get “page cannot be displayed” (I still have a live wired connection at this point)

Once you restart you can browse the internet again for about 5 mins.

I have done a handful of virus scans using different software and nothing shows up. I also checked my host file table. Checked power settings to.

My next step is going to be replacing the network card

Any suggestions?