iChat CPU Usage

In the last few weeks iChat has been surging to 70% - 90% CPU usage, staying there, and causing the fans on the G5 iMac to kick in.

This is, obviously, rather irritating, and I have attempted deleting preferences, restarting the system, terminating the application and restarting it, etc. None of this seems to precisely fix the problem, if it does anything at all. The only thing that seems to be consistent is that the surge in CPU usage seems to occur with greater frequency at late hours, 11 PM to 1 AM (Pacific Time).

Any thoughts on why? If not a solution?