The following was taken from this URL http://www.cisco.com/univercd/cc/td/doc/product/wireless/airo_350/350cards/windows/incfg/win5_ch8.htm :
"PLCP CRC Errors
The number of times the client adapter started to receive an 802.11 physical layer convergence protocol (PLCP) header but the rest of the packet was ignored due to a cyclic redundancy check (CRC) error in the header.
Note CRC errors can be attributed to packet collisions caused by a dense population of client adapters, overlapping access point coverage on a channel, high multipath conditions from bounced signals, or the presence of other 2.4-GHz signals from devices such as microwave ovens, wireless handset phones, etc."
You will need to check to see if there is anything that would be causing interference. If you do not see anything physically that could be causing interference, try changing the channel on the access point to anything other than it's default channel 6. If you are running an old version of firmware on the AP or the client adapter you might want to upgrade to the latest version. Check your version of ACU to see if it is out of date as well.
