I have run into the same problem. I am/was using a linksys wireless gaming adapter to connect my 360 ot my router. It worked great for some time, then all of a sudden it was garbage. Got a new router and for about 1 evening it was ok. After that it was garbage again.........after messing around and trying to find the problem, one day i pick up my cordless phone and can listen to my neighbors(apartment). after asking them, it turns out that the old lady has 4 cordless phones.....my wireless network is now garbage thanks to 2.4 ghz wireless phones. Changing channels doesn't help at all, so i am left to just eat it. That is a pain.
Right now i am running 50 foot of Ethernet from my upstairs router to my 360 downstairs........it really sucks.