Some thoughts that came to my mind:
are the affected ssids tunnelled or local?
If tunnelled, is the Datachannel Security TLS or IPSEC?
Are Protected Management Frames enabled on the ssid?
Does it also occur without WIDS on the AP Profile?
Have you checked the AP´s CPU Load?
Unfortunately I have no 221C in reach to test it myself.
321C´s are working fine here.