Network Card sleeping - Check if your network card is allowed to be turned off when your computer enters power-saving mode. (Computer Management, Device Manager, Network Adaptors, Your network card, right-click-Properties, Power Management tab, Unckeck the allow the computer to turn off this device checkbox.) I've had instances where windoze keeps on turining the nw card off if some wierd screensavers are installed. Bizzare, but worth a look.
If you switch back to non bridged mode, you can consider giving your pc a fixed ip address, like the one that it would be assigned any way, and then adding that address to the DHCP table on the router as a permanent address.