Absolutely unacceptable CPU usage during video calls on a Mac

During video calls on my MacBook (OS X 1.5.6), CPU often hits 100 or higher for Skype. Happens using the beta or the most recent version, only during video calls. Generally, I have to put the person on hold and off again, and that fixes it for a few minutes.

Symptoms include extremely laggy video, frequent freezing and much worse video quality than anticipated. (I have high-speed internet, and my MacBook is 2.16 GHz Intel Core 2 Duo with 2GB of memory.)

Probably related but possibly not is the accompanying audio choppiness, which is sometimes fixed (I think) by plugging in and unplugging headphones.

Before you suggest it: I have already done a total wipe of Skype and reinstalled, including clearing cached files. Would you like a sample from Activity Monitor? What can I do to help solve this? It's really irritating.