Though OS X should, under normal circumstances, just automagically figure all this stuff out on its own, certain brain-dead networks are messed up to the point that standard settings won't work reliably and OS won't be able to auto-correct.
In particular, some (not me, thankfully) will have a problem with their Maximum Transmission Unit (MTU) set too high. There are some older hints discussing this problem, but none really discuss how to figure out if you do or not.
This script will do the trick. If it says that your MTU is 1500 or more, you can forget all about MTUs. If it says it's something smaller, and you've been having network problems, you've probably found at least one of the mysterious sources of your troubles. If it says it's something smaller, and your network is behaving properly, just forget you ever ran the script.
[robg adds: Save the script using your favorite text editor, and then remember to make it executable (chmod a+x script_name) before running it. You should also edit line 13 (PingHost="ns.fastq.com") and replace the hostname with a host of your choosing -- your local nameserver is suggested in the script's comments. I tested it, and it seems to work as described.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060201155734147