Its crap having an intermittent internet connection. I too have been experiencing my ADSL connection is disconnecting itself every now and then. And My network isn't a simple wireless router either.
Is Linux picking up an ethernet connection - check?
Is the DHCP server working - Check?
Can I ping and/or login into the router web pannel - check?
Can another computer access the internet - check?
Have all switches been through a power cycle?
Are all cables secure and picked up by switch - check?
IS telephone cable ok- check?
Tried the master socket - check?
Then as a last resort use tech support. go through everything, even if you have done it twice. They are bound to give you a new idea to try and in past they have been correct

. I thought it was BT's fault, but it was partially theirs as it was their telephone line.
Anyway, hope you can get the issue resolved ASAP. We'll miss you.