I think you have a router problem, too. Make sure you get the updates for it, as well. Apple will find it and prompt you to update the Firmware.
But, I did experience this exact problem on a Mac Mini, after the upgrade from Leopard (10.5) to Snow Leopoard (10.6), as well.
I suspect a problem or corruption in the PREFERENCES. This has always been the weak link in the Mac OS, even on pre Mac OS X versions. It's not serious, but can be annoying.
I use Tech Tool to reset the PRAM and that forces the system to reload all of your Preference settings for the computer and all the applications your run.