Just a disclaimer to say I've never done this exact setup, but I've dealt with some occasional glitches on internal networks, and this sounds a lot like the problem one gets when one is trying to connect to servers inside a network that is using Network Address Translation to route packets from the external network to the internal network. So, for example, say computer A in network 1 is talking to a computer B in network 2 and network 1 is an internal network, because B sees network 1 as a single IP address, it can't connect to computers in network 1 unless the router linking the two specifically knows which of the internal computers to forward the packet to on that port. However, computers from network 1 can send (and initiate connections) to network 2 and have packets sent back to the right computer
If you're seeing that kind of behaviour, then somewhere, you or the firewall software screwed up the routing. You can have a look at how the kernel is routing using the route commands, or the ipchains command to view the Network Address Translation part of the Firewall. (if you go down this path, expect to do a lot of reading up about the commands).
Also, as a side point, it seems like a bit of an overkill to do a DMZ for a home network. Most home networks benefit from the assumption that if someone is inside your network, then they're probably inside your house, or hijacking your wifi. So it's far better to put stout locks on your doors, WPA encryption on your wifi and a good firewall on the internet end of your connection (between the server and the internet) and assume that your internal network is secure otherwise. So are you sure that a DMZ is really what you need? I also think a DMZ would be really hard to set up without using at least three computers (or one computer and two routers), and you sound like you're using just one.
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If you're seeing that kind of behaviour, then somewhere, you or the firewall software screwed up the routing. You can have a look at how the kernel is routing using the route commands, or the ipchains command to view the Network Address Translation part of the Firewall. (if you go down this path, expect to do a lot of reading up about the commands).
Also, as a side point, it seems like a bit of an overkill to do a DMZ for a home network. Most home networks benefit from the assumption that if someone is inside your network, then they're probably inside your house, or hijacking your wifi. So it's far better to put stout locks on your doors, WPA encryption on your wifi and a good firewall on the internet end of your connection (between the server and the internet) and assume that your internal network is secure otherwise. So are you sure that a DMZ is really what you need? I also think a DMZ would be really hard to set up without using at least three computers (or one computer and two routers), and you sound like you're using just one.