Apple has released an OS X update specifically for the iMac called (oddly enough) iMac Update for OS X. The update includes fixes for networking and more. This from Apple:
The iMac Update includes enhancements to support the installation of third party applications and the Networking Update which improves network and Internet access after restarting your computer or when waking from sleep.
You should also note that Apple has targeted this update for iMacs using a particular build of OS X. From Apple:
Important Information:
This update is for computers using Mac OS X 10.1.5 build 5T91. To display the build number, choose About this Mac from the Apple Menu and click on the line with the Mac OS X version number -- "Version 10.1.5". The build number appears.
You can grab the update from Apple KnowledgeBase website.