I think I know what might be the problem If I'm reading this correctly.
You're netgear gs605 is called an 'unmanaged' switch. This means just plug in your cables and leave it alone. But, unless the piece of equipment connected to it has the settings (Duplexing etc) to make the throughput (speed) 10MBs, 100Mbs or 1Gbits connect at one of those speeds, you may get this jumping from one to the other.
The Xbox seems not to have a networking setting to allow this, and therefore it would appear it is moving from 10, 100, 1Gb. This can cause many problems. Which is one you are suffering from.
Unfortunately the switch you have is not an 'smart' switch. Which can stop this happening, but they are expensive.
I found this about connecting to a network, which includes using avplugin units;
http://www.microsoft.com/windowsxp/using/networking/...
That might help.