Looks like Linux 3.8 has a stupid bug. The e1000e driver for Intel NICs does not work correctly (ref. here). In short: The powersaving prevents the NIC from comming up correctly. So I wrote a udev rule to always power on my NIC. In fact there are only a little chances I won’t need it:
SUBSYSTEM=="pci", ATTR{vendor}=="0x8086", ATTR{subsystem_device}=="0x21f3", ATTR{device}=="0x1502" ATTR{power/control}="on"
Compare vendor, subsystem_device and device with your own configuration. Either with udevadm(8) or with udevinfo(8).
Another problem with powersaving was my mouse. Everytime I didn’t plugged in my powercord, I had a very stuttering cursor. So I wrote another udev rule:
SUBSYSTEM=="usb", ATTR{idVendor}=="1d6b", ATTR{idProduct}=="0002", ATTR{power/control}="on"
It is nearly the same except of the “search attributes”.