I know this might sound stupid, but did you upgrade to the latest firmware? It might solve some issues.
About your QOS, I'd leave it alone. The reason why? Well here's my setup:
4 wireless devices (2 which watches IPTV all day long, 2 so..so..
1 Voip system, 1 roku box streaming HD netflix, 1 desktop playing starcraft II, wow, etc, and a file server which streams HD content occasionally. All the wireless devices and desktop are set to static ip, but the voip, and netflix streamer are set up with DHCP.
All of that with a 6Mbps DSL. So if I can do it, I know you can definitely can.
Last question, where is the router position? I've got my 2(two) DIR-628 routers, 1 is the main router with wireless turned off, and the other is acting as an access point which is position as close to the middle of my house. Since being in the center, the signal is evenly distributed and all my wireless devices are getting an equal share of the wireless signal.
Oh by the way, do you live in or near an apartment or condo? The reason why I ask is because, as technology becomes cheaper more and more people have them. The wireless scene might be getting crowded, so you might have to choose a channel that no one is using. For example, I'm using channel 4, which I am the only one on. Everyone else is either on channels: 1,6, or 11. With those channels crowded there can be lots of interference because everyone is sharing the same channels. Your signal becomes noisy and its integrity is up to par. @optimum That might be why you aren't getting a solid signal.
I hope this helps:)