I don't have enough reputation to comment, so I'll ask some questions here along with some suggestions...
What type of connection do you have, and what speed is it rated for? Does Netflix buffer (load) frequently?
Some generic suggestions without knowing these things:
- If you get buffering fairly regularly, odds are you either have too low of a connection (a 1.5Mbps DSL line BARELY runs Netflix), or you're saturating your connection with too much stuff at once (do you have any large downloads running?)
- Your router could have trouble keeping up with the traffic, due to faulty firmware, lack of resources, etc. I'm not too familiar with Belkin routers, so I'm not much help there
- If you're on a cable line, it's possible you adding a Netflix stream is just enough to saturate the "neighborhood" pipe (bandwidth is shared between a group of neighbors, so 5 people with 20Mbps lines might be sharing a 50Mbps pipe)
As for QoS for Netflix, you're probably out of luck. From my research for a similar problem (I couldn't use the internet while watching Netflix or it would start buffering whatever I was watching very frequently, due to a small DSL line), Netflix streams via HTTP. Granted, I never actually tried sniffing the traffic, but everything I found online pointed to that.
Finally, a "related" question got an answer related to a bug in dnsmasq used in DD-WRT and other custom firmwares, but the OP never marked it as solved. The OP in that question WAS using a similar router (A Belkin N150, although no model was listed), so I'm thinking a firmware/hardware bug may be at the root of it. You should look at updating your router's firmware, or possibly flashing a custom one if one is available (I personally use DD-WRT). If you have the ability to try another router, try that if an updated/replacement firmware doesn't help. If that's not an option, you can find fairly cheap replacements on Amazon or Newegg (the Asus RT-N10+ is currently $25 on Amazon, and supports DD-WRT...that's the one I'm currently using).