I am having the same issue, but it also happens from time to time with a wired connection.
Look back at your logs after this happens and see what the router did to the client. It will list an action along with a reason code. The reason codes are listed here: http://www.cisco.com/en/US/docs/wireless/controller/3.2/configuration/guide/c32err.html
I continuously get reason codes 3 and 8, and neither makes any sense.