I was having similar problems with my Sony Vaio VGC-LT1M. The machine would drop off the network and needed to be switched off and on again - a simple reboot would not work. Sometimes it would just reboot itself part way thru a download, especially when updating tv listings from DigiGuide UK. It would also do it when transferring a file over the local network or viewing a MPEG video file. I have an Atheros chip in a LAN-Express AS IEEE 802.11g PCI-E Adapter. Tried all sorts of things, updating driver etc. What seems to have worked for me is switching off the QoS Packet Scheduler - double click network icon in task bar, click properties and untick the QoS box. Hope this helps - it did it for me!