I tracked down the following Microsoft Knowledge Base article "Network connectivity may fail when you try to use Windows Vista behind a firewall device
" which describes the problem I was having under Vista.
It describes how to temporarily turn off (and restore) the Receive Window Auto-Tuning
feature so you can troubleshoot. This worked for me until I was able to upgrade my router's software. By the way, the Knowledge Base article describes opening a Command Prompt window to implement the fix but, to my surprise, it doesn't specify starting that Command Prompt window as an Administrator which would be my recommendation.
Just in case you think the Knowledge Base article doesn't describe your symptoms then check out the tech-recipes
article which starts:
In Vista TCP Window Scaling is enabled by default. If your router does not tolerate this feature, it can cause random DNS and connection errors. This is a common cause of vista limited connection errors.
Anyway, it costs nothing to try the fix except a slight loss of TCP tuning and it is esy to put Vista back the way it was when you are done.