Network Connection Failure

I have a windows 2003 server that is running Exchange 2003
on it, it has two identical network cards built in to it,
every few days the Internet Connection (card 2) goes down
while the other card continues to work perfectly.

now when the Windows 2003 builtin firewall is running it
goes down every 6 hours or so, when it is not running
every 30 hours or so....

if the network connection is disabled and then enabled
then it works fine again for 6 - 30 hours....

durring this time the Internal Network (Card one) never
has any problems at all.