After a fairly painless upgrade from Tiger to Snow Leopard, I was able to download a few things. Then I started having difficulty connecting to multiple sites. Network service would come back from time to time. I eventually figured out that some sites would work because they were dns-cached, while new sites were failing at the DNS lookup. I have another computer on the same LAN, so I know it's not an upstream problem.
The DNS server behavior changed in Snow Leopard. I had a custom network location set up with an additional DNS server (my router, so I can use network names for my two computers) In 10.4/Tiger, this server was queried in addition to the DHCP servers. in 10.6 Snow Leopard, it is queried instead of the DHCP servers. I had no trouble when switching back to the