I think there is a bug associated this. The bug ID is CSCin18914 and you can find this in Cisco's Bug toolkit.
Also, make sure that you have not turned on Publicly Secure Packet Forwarding (PSPF). But if PSPF is enabled, the issue will be permanent.
Since this occurs sometimes only, there are possibilities of something going wrong on the RF side.
You can also try upgrading to the 12.01T firmware if you are running some older version.