Problem: After (I think) an update my Mac began to show a strange pattern of network latency: A ping to my router would show a string of 1ms trip times punctuated periodically by a cluster of 400-1000ms latencies. I first realized something was wrong when I was playing counter strike and my ping would go crazy - on a server I frequent my typical ping used to be 20ms, but now it would register as 190ms, 400ms and so on. I could verify this was not a internet provider issue by pinging my router from a different computer and getting a steady string of 1ms latencies.
The solution was remarkably hard to find even though it seemed to be a common problem faced mostly by Mac users. A common response was to turn off anti-virus software, and I knew this was not the correct answer.
Solution: From the thread here (look for the answer by "eyepaq") it seems to be a bad setting or combination of settings in network preferences which can be reset by deleting the file "/Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist" and rebooting.
I was a little surprised at how happy I was that I could play CS:S again - not that I play so much, it was just such an annoying problem.