OS X V10.7 Lion :: Losing Internet Access For IMac - WiFi Icon Changing To Exclamation Mark

For several weeks I've lost internet connection on my iMac every day or so; the wifi icon changes to show a ! and is otherwise greyed out. My internet connection is via a relatively new Time Capsule (2011) which has an ethernet cable connecting it to my Sky broadband router. I moved to this new Sky broadband router in early Jan 2012 (changing to Sky as a Broadband provider) and it works nicely as a router. My iPhone, iPad, AppleTV and a range of other devices (including an Apple Airport Express connected to a Sky TV HD box to enable it to download movies) have continued to work on the internet without problems.

The IP address of the iMac (27 inch quad core i7, 8 GB RAM, fully up to date on Lion) disappears from the Network settings when the problem arises but the connection of the iMac to the wifi (and backing up to the Time Capsule) still works. I can even use a VNC app on the iPad to connect to the iMac on the same wifi LAN when the IP address has gone and the ! is showing in the wifi icon on the menu bar. To get reconnected to the internet, I just restarted the iMac and it was then back accessing the web nicely. Eventually the iMac would not connect to the internet at all. It would momentarily get an IP address from the router and then lose it again. I then tried taking the iMac to the Apple Store.

Various diagnostics run in store at Genius Bar and during the week and the problem (using the same account) was not replicated. On its return home, it was still not working on the internet. I then tried booting into 'safe mode'. This enabled a stable and reliable connection to the internet! After several days in this mode, I re-installed Lion and booted into normal mode again. The problem returned and has been with the iMac since. However, at least a restart of the iMac gets it straight back on to the internet again. I suspect some sort of corrupted network preference file or the like. But where? The disconnection from the internet can happen any time of day; for example it happened while typing this message but more often than not it happens when the computer is unattended.

Info:
iMac, Mac OS X (10.6.7)