If your using a router you might want to try pulling it out of the equasion. I was having the same problem, with my linksys wireless G router and when i switched it back to my older wired router i had no problems. I had portforwarding setup correctly and everything, I've emailed linksys about this numerous times and haven't gotten a reply, i've since switched to a belkin wireless router without any problems. Hope this helps.
Stupid people are like Slinkies, it's fun to watch them fall down stairs. And some times you need to give them a push.