I've tried the last several days of the Ubuntu 13.04 daily builds with the same result on booting the live DVD of each. Starting with the selection screen for Try Ubuntu or Install Ubuntu with my ethernet cable plugged in the Network Manager engages in a continuous and unstoppable cycle of connecting and disconnecting. This happens whether my WiFi is turned on or off. If I select Try Ubuntu and go to the desktop it still remains stuck in this continuous loop. I've tried to change settings to no effect. Up until about 5 days ago the daily builds booted and installed without suffering this effect. Maybe it's kernel related?