what are some ways of fixing inconsistent internet issues

Im working on a home built PC running windows XP Home Sp3. It is connected to a DSL modem. the internet works great for the first 5 to 10 mins then it starts saying that the page cannot be displayed. if you do an IP /release then /renew it doesnt help. you have to cycle the power on the modem. then the internet will work again for 5 to 10 mins. One of the odd things is that when the internet wont work the INTRANET will, if connected through a hub i can connect to other computers on my lan but not the internet.

I ran several AV / Malware scans and found nothing. I also ran a winsock fix tool. my system works fine on the same modem so i dont think their is a problem with the modem.

Anybody had this problem before?
or
Any ideas?

Thank You!