IMac WiFi connection to the router is unstable

Try using iStumbler to detect and identify neighboring and overlapping wireless access points (WAPs). It's likely an interference issue that can be solved with vigilance. You need to find out what channels your neighbor's WAPs are using, and then switch your router to use the least crowded channel you can find.

Ideally you want to use channels 1, 6, or 11. These are the channels that have the most leeway when it comes to other channels interfering with their signal. Wifi devices operating on a particular channel can interfere with other devices operating on channels up to 5 steps away. The amount of interference grows logarithmically with signal strength and channel proximity.

For example, if you are using channel 1 and your nearest neighboring wireless network is also using channel 1, you are receiving a large amount of interference. If you are on 1 and the other network is on 6, the interference is negligible. However, if you are on channel 1 and your neighbor is using channel 3, you are likely still on the receiving end of a huge amount of interference depending on their WAPs signal strength.

iStumbler will give you information about the signal strengths emanating from the other interfering WAPs, and from there you can infer the best channel to use. You may need to periodically perform the same task as your neighbors change their wifi channels.

Mac Wireless & Airport Connection Problem Troubleshooting: The Basics * Turn Airport on & off – You can do this via the Airport menu bar or from the Network Preferences. This is the first thing you should try when troubleshooting Mac wireless problems.

• Reset your router – This is the second thing you should try doing. You can fix a surprising amount of wireless problems just by resetting the airport/router. All you need to do is turn the thing off for a few seconds and turn it back on.

• Reset your Cable/DSL modem – You’ll usually want to reset this in combination with your wireless router. Reset this first so the DHCP information will be pulled to the wireless router properly.

• Change Wireless Channels – sometimes your router’s wireless broadcast channel will interfere with a neighbors, be sure you have your router set to a unique channel. Even if it’s a weak signal there can still be interference.

• Make sure Wireless/Airport card software & firmware is up to date – This is usually done just by going to the Software Update menu, if there are any updates available for your Mac or Airport, install them.

Mac Wireless Troubleshooting: Intermediate * Change wireless security protocol – You shouldn’t be using WEP anyway for security reasons, but sometimes changing from WEP to WPA/WPA2 or WPA to WPA2 can resolve wireless connection difficulties.

• Make sure router firmware is up to date – Check your router manufacturers website for firmware updates, if there are any available, install them.

• Delete and recreate connection – Try deleting and recreating/reestablishing the wireless connection, sometimes a setting can be corrupted and this may fix it.

• Create a new Network Location – Similar to the above suggestion, try creating a new and different wireless network location to see if it resolves the connection problems.

• Change DHCP auto settings to manual – sometimes there is a problem with the DHCP server, and if you manually set an IP address on the network you can be fine. Remember to set the IP to a high number so it wouldn’t interfere with other DHCP machines. As long as you have the subnet mask, router, and DNS settings configured manually as well, this shouldn’t be a problem.

• Disable “Wireless G/N/B only” mode – Sometimes a setting is selected that only broadcasts your wireless signal in Wireless B, G, or N mode (depending on the routers abilities). If this is set, try disabling it.

• Flush the DNS cache – Launch the Terminal and enter the following command onto one full line within the Terminal:

  dscacheutil -flushcache
  

Mac Wireless Connection Problem Troubleshooting: Advanced * Zap the PRAM – Reboot your Mac and hold Command+Option+P+R during restart until you hear another chime, let the Mac boot as usual.

• Delete Wireless Config files – Delete com.apple.internetconfigpriv.plist and com.apple.internetconfig.plist files from ~/Library/Preferences and reboot

• Trash your home directories SystemConfiguration – Remove all files within ~/Library/Preferences/SystemConfiguration/ and then reboot your Mac.

• Reset your Mac’s System Management Controller (SMC) – For MacBook and MacBook Pro’s: Shutdown the MacBook/Pro, remove the battery, disconnect the power, hold the Power Key for 15 seconds. Replace the battery, reconnect power, and zap the PRAM and wait for 2 chimes before letting the keys go. Let boot as usual.

source: osxdaily.com http://osxdaily.com/2009/12/22/mac-wireless-problems-guide-to-troubleshooting-airport-wireless-problems-on-your-mac/