Internet problems on one Mac, other is fine

For the past 5 months, my 27" 2011 iMac has been plagued by problems connecting to the internet or maintaining that connection ... but my 2009 Macbook connects to the same home connection just fine.

This issue with the iMac persists on both wired and wireless connections as well as in Mac OS and in Windows through Bootcamp. Right now, my iMac has spent the last 15 minutes struggling to connect to the internet (all other applications closed except Google Chrome) after restarting the machine, but my Macbook connects just fine seconds after waking it from sleep.

This leads me to believe there's a problem with the iMac itself, but I'm at a loss for what the issue could be. Any ideas?