Did you setup your macbook from a time machine backup of a previous mac? If so, there might be a rouge app that might be causing it.
It took me almost a month to find an old USB internet dongle app carried over from my old WhiteBook that was causing my more recent Macbook Pro make our entire home network disconnect from the internet almost every 15 minutes. It only showed up in the very bottom of the System Information report, and not in System Peferences >Users and Groups > Login Items.