I've been wrestling with a weird network problem with Mac OS X Leopard (10.5) for the past couple of weeks. It looks like part of the problem may be related to the way Leopard handles DNS queries. I still haven't found a fix, but it looks like...

OpenDNS

...is a decent work-around for the moment. OpenDNS lets you use its fast DNS (Domain Name Service) routers in place of your ISP's. They claim they are faster (and that seems to be true) and safer (because they block phishing sites and let you filter out additional sites). You can find a longer review and description of OpenDNS in this ComputerWorld article by Preston Gralla.

Hack DNS for lightning-fast Web browsing