I had this problem with 138xx and am still having this problem with the latest build. I have QoS enabled, have the MAC address of my Xbox specified to have premium bandwidth. Now when the Xbox is on, the internet speed for other computers slows down which is what I want. However, when the Xbox is offline, the speed for the internet on other computers is still slow even if I remove the Xbox from the DHCP list. Is this a bug or do I have it configured incorrect?
http://www.jkogden.net/media/Capture.PNG
I multiplied my bandwidth by .85 to get the results in the corresponding fields.
Change your packet scheduler to HFSC, that will fix it.