Broadband connection is always down

I have a broadband connection that is connected to my ISA
server and my ISA server is connected to my local area
network. At first, I can browse the internet but after
some time my connected seem to be down and all the page I
go give the "page cannot be displayed" page. Then, I need
to go to my ISA server to disable the network adapter that
is connected to my ADSL router and enable it in order to
have the internet connection again. My ISA server O/S is
windows 2000 and it have two network adapter one is
connect the ADSL modem and the other one is used to
connect to my Local Area Network. The ADSL modem's model
is Aztech. This problem has been buging me for sometime, I
really hope that there is someone who can help me. Thanks