Ubuntu 13.04 is getting boot and power off problems

Ubuntu 13.04 is getting boot and power off problems

Hi ubuntu ask community:
My problem is: sometimes i boot my PC, it gets stack in the black screen
with white letters saying something how:
IPv6: Link doesn't stablised.
....: Link doesn't exist.
And when we power off i like something how i wrote above.
Today i get only that error, i restart, and error, i disconnect my USB
Wifi, and it boots.
But i want i solution, before i haven't any problem with Ubuntu.
Thanks.