Dropping Internet Connection aftere few Days

I have Windows SBS 2003 & i have installed Exchange Server, DHCP, DNS & Active Directory.
Let me explain my network. I have a Cable modem & then i have Linkysys Wireless router and Wndows Sever 2003 and work station & labtop. Everythig is fine and after few day i am not able to browse internet. Then i have to re-boot my server and everything is okay.

Can anyone tell me what wrong.

Thanks in advance
Shri !!