There are at least three similar bug reports (including this one) here in Launchpad on this problem:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1144322
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1165433
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1189998
There are at least three similar bug reports (including this one) here in Launchpad on this problem:
https:/ /bugs.launchpad .net/ubuntu/ +source/ linux/+ bug/1144322
https:/ /bugs.launchpad .net/ubuntu/ +source/ linux/+ bug/1165433
https:/ /bugs.launchpad .net/ubuntu/ +source/ linux/+ bug/1189998