I'm glad I found this thread because I was beginning to think that I was having hardware issues with my new motherboard.
Just to give more datapoints, I'm running Build 7100 and have also noticed these strange network drop-outs. I've had two different types 1) intermittent short 1 or 2 seconds hiccups where requests will hang and ping packets will either drop or had unusually long times and 2) issues where heavy network traffic will just cause the network connection to completely drop causing gateway errors and giving the yellow exclamation point on the taskbar.
It seems, and it's hard to say for sure, that this started once I did the windows updates and pulled down all the critical updates. I had the machine running for about 3 days with no issues whatsover. However, the problem is so intermittent that I can't be sure it wasn't there before and I just didn't notice it.
I have also done alot of troubleshooting including removing my WRT54G from the picture and connecting my machine directly to the modem. I have been able to see issues both on my LAN and directly connected to the WAN. At one point, I was able to consistently cause the connection to drop (i.e. yellow exclamation point) by running a bandwidth test on SpeedTest.net. The connection would come back after a minute or two and then another speedtest would cause it to drop again.
For the first situation I mentioned above, I actually had to reboot the machine to correct the problem (the hiccups but not lost completely problem). For the second problem, I can just disable and re-enable the network adapter.
So far, I have tried disabling IPV6 and disabling all power server/WOL options in the card. Nothing, so far, has made a difference.
-Aaron