What is eating all of my CPU?

A few months ago my development workstation running Feisty started exhibiting very annoying behavior when debugging a web app running on Jetty with the Eclipse debugger. I don't know what changed, but now, whenever I stop at a breakpoint, my CPU becomes 100% in use. I can still use the debugger, but it is *very* sluggish. I was expecting it to be either the Eclipse or the Jetty jvm hogging all of the resources, but to my surprise, 'top' tells me that it is Xorg that is eating all that CPU!

Note that if I debug a simple java app or a junit test, this problem does not exist.

So, why Xorg? What can I do to get a better understanding of the problem? Any ideas would be greatly appreciated.