(1) If you've not already tried, restart Mac and test again with no apps except iChat running.
(2) If problem persists after restart, use Disk Utility to repair permissions.
Then download and apply the 10.7.4 Combo Update:
Repair permissions again immediately following the system restart that completes the Combo update.
Properly applying the Combo version of updates can sometimes correct software problems that might be causing your trouble.