Routing problems on gateway affecting LAN

[Arny]

I've got a fairly standard home server setup as a gateway/firewall serving a few Windows machines on my home network. Dnsmasq takes cares of the DHCP and DNS duties and an iptables firewall script takes care of any port forwarding and other firewall duties. I have a redirection to Squid for all traffic over port 80. As of last week I'm unable to use either Steam or Guild Wars to login or download any updates when connected to the internet through my gateway server, but I am able to connect to them if I take the server out of the equation.

I haven't noticed any other problems connecting through the server and the only thing that has changed recently is my ISP, which changed the day before my problems started. I'd automatically suspect them of blocking ports except for the fact that everything works fine with a direct connection (i.e. no gateway server).

I think it's something to do with the routes on my gateway, but I can't be sure. Does anyone have any other thoughts on what could be going wrong here? I'm happy to provide any additional/more specific information. For starters here's the routing table:

[Arny]

Apologies for the double post/bump.

Some extra information — I'm able to install Guild Wars (which involves downloading updates) on my work laptop when I connect to the network wirelessly and the internet is routed through my gateway. The only difference from a networking perspective is that my work laptop is a member of a domain, whereas my other systems are all a member of the HOME workgroup.

Anybody? I'm totally lost on this one.