Linksys AM300 Modem + Strange Behaviour


In Runtime
Hi guys.
A few weeks ago we had a thunderstorm and heard lightning strike close to our home - luckily my PC was at a friends place, but whatever happened seemed to have killed our old Netgear Modem-Router. It also blew my mum's onboard network card into non-existence - everything on her computer is working fine except that the ethernet port has its orange LED all the time, and her computer doesn't detect any network hardware in its device manager.

This isn't really the problem, though - just the backstory. Since then we purchased a new ethernet PCI card for my mum and its working fine, and we also got a Linksys AM300 ADSL Modem. Our computers connect to a Netgear Wireless Router (non-modem), which is connected to the AM300.

The AM300 worked fine at first, after fixing up the settings. We even had it connected to my mum's PC via USB (we got the modem before we got a replacement LAN card) and to the router via ethernet simulteanously, and it had no problem. However, the next day we woke up to find the internet not working at all. After a few hours it started to work again, but today it stopped again with similar symptoms and it's starting to annoy the hell out of me.

Here are the details:
- If I try to go to a page, Firefox goes through "Looking Up", "Connecting To", but then sits on "Waiting For". On some occasions it sits on "Looking Up"
- If I ping a website via the command prompt, sometimes it DOES receive a response
- If I ping or do a traceroute from the modem's settings, sometimes it DOES receive a response
- If I troubleshoot on Windows Live Messenger, it tells me that there's a problem with the KEY PORTS, and sometimes with the DNS.
- If I change any setting on the modem, the internet will work again for 10 or so seconds.
- The internet LED on the modem lights up fine, and the modems status page shows that everything is working OK.
- If we connect a computer directly to the ethernet port, or via USB, the same symptoms occur.
- The issue occurs with all 3 computers we have on the network.

I would appreciate any help you can offer me.