Client cannot access internet but intranet is ok...

Dear all

I've the following problem, please help if you've any idea..

1. Using DHCP (Winodws 2003 server
2. Cisco route
3. two XP pro clients (with autoupdate enabled) connected to a small switch, then to the core switch, check and replace all switch/port/cabl
4. both clients do ping -t 192.168.1.1 (default gateway/router IP address
5. sometime one of the client cannot be accessed (request timeout), another are fin
6. repair the LAN connection (right click the LAN connection icon beside the small clock
7. the connection is resume to norma
8. but it will happen again later..

Please help me, thanks for all the expert

-
Frankie