Connecting to the internet problem

Hello,
im using a d-link modem. when i on my computer the network says im connected to local lan and the internet but after 10 minutes (varies from 10 minutes to 1/2 hour) it goes offline and i cannot acces the internet. net work says im only connected to local. if i open my 192.168.1.1 setup it says im connected to the internet but i cannot browse. i only can resolve this problem by rebooting the modem but it after some minutes i cannot browse the net..dsl light on the modem is fine as normal. i using my other modem it seems fine, no offline issues. is there something wrong with my modem. can i sent it to a shop to get it fix.