cannot connect to internet

i use cable modem Scientific Atlanta Webstar dpc2100

when i connect on turn on PC it gets addresses from ISP but i can’t open any page.

i restart my modem firs it gives local addresses when it get ip from ISP and for about 10sec i have internet and when again it gone.

i couldn’t find what might be blocking it – tried to run with all my firewalls, anti virus temporally off – same. what else i can do?