ummm... you're monitoring your airport signal strength off the menu bar, right?
Hell, that won't make a difference, I don't think. With my 12" G4 iBook I can scab bandwith from a couple of neighbors and iChat doesn't crash on me when my signal wavers.
Using iChatAV and Panther of course, because both shipped with the G4 iBooks.
Try elimanting factors and start with the obvious. Run a cat5e from your cable modem to your iBook and see if still crashes (which it probably will). Elimante all the network aspects of the problem.
Hell, I dunno. Anyone else?