I have been having this problem for a while now to the point I would disconnect a couple times per hour. I know its not my connection because my sons play the same time I do, on my network and they stay connected. We all randomly dc with errorcode 9000...sometimes all of us at the same time...sometimes only one of us. I did turn off the UPnP on my router and the dc has been reduced drastically though...maybe 1-2x a day instead of 1-2 an hour...just fyi
From your information, it is something to do with your connection. Especially since you said changing a setting in your router pretty much nulls the errors.