Our office is set up with mostly macs (7 of them) but we do have a windows laptop and a windows desktop on the network as well. The network is configured with a modem going into a switch/router throughout the office to the computers, along with a wireless router. Everything runs fine most of the time, but periodically while using the web, certain sites will stop loading and timeout repeatedly. This usually lasts 20 minutes or so and can be incredibly annoying. Resetting the modem/router and/or rebooting the computer never helps. The weirdest part is that in almost every case, the websites are fine on our Windows machines. I frequently use github, google, Stack Overflow, and jQuery reference and I can count on the sites being unavailable to me at least once a day. While I can't get them to load, I can spin my chair around to the windows server behind me and load the sites just fine. Any idea what the hell could be going on here?
SnapOverflow
If you are using residential routers such as Linksys, I've encountered issues due to TCP connection limits being low. If you cannot increase the connections with the default firmware, DD-WRT allows the connections to be modified.
If your troubleshooting eliminates all equipment within your office, it may very well be your Internet connectivity. You could work with your ISP to troubleshoot that connection when the issue is occurring.
Take a look at your DNS settings as well. Are they different on the Macintosh machines than they are on the Windows machines?
I have encountered a similar issue once at a SOHO where there was interference from a nearby wireless access point. The wired users had no issues.