Well, basically I've given up and have started to use Hamachi.

So, I used to be able to host a server fine in the spring, didn't really do anything in the summer, and then started to do it again in the fall. In autumn, however, my friends could no longer connect to me. My ports were "open" according to my router and all, and they tried connecting via my IP and (if over at my house my DHCP IP) but still couldn't connect. I could do it myself because I just used my loopback.

Today my girlfriend bought Minecraft so I wanted to get it to work to teach her the game and play with her, but it still wouldn't. I made sure all router settings were as they should be, and even upgraded my firmware, yet still all the port tool sites would say my ports are closed when I checked them.

My ISP is Cox, and apparently to all known sources they don't block the ports in question (they only do a handful). I use a Linksys WRT54G and I believe my modem is by Motorola.

Does anyone have a clue to what might be the underlying issue here?

Faulty router would be my best guess. There was a spare one here when we moved in so I might try to see if that provides different results than the Linksys.

Any help is appreciated.