i'd suspect it's something with your setup, piggy backing off of the mac, i've never been able to make that work. i use an ethernet Switch (not a router) and that works like a charm. internet goes direct into the first port on the switch, then one cable goes to my Dell, another goes to my 360, and a third goes to my mac. never had any issues
switches also aren't too expensive, i've got a d-link switch, cost me ~40$