Update the Intel drivers on the client to the most recent.
Also note that two MIC failures in 60 seconds can (did, in my case) shut down the radio.
The MIC failures are viewed as an attack, so by spec, the radio is shut down to prevent entry (I don't know how long it drops, but I suspect it's ~ minute or two).
The CWNP book says some / many / most AP vendors will provide an option to ignore this error because it could be used as a DOS attack. I haven't found (haven't looked) to see if Cisco has this parameter override.
I had this on my Lab network with a WDS of a couple AP1200s. One of the clients was running an Intel Centrino NIC, was throwing MIC checks, and killing the associated AP1200 (running 12.3(4)JA)).
FWIW
Scott
