For unknown reasons, the menu plugin for the xfce4 panel has suddenly started to consume all free CPU cycles for this machine, coincidentally rendering the menu button completely unresponsive for the duration of the bug. 

This will happen about every 10 to 90 seconds and last for about 5 to 10 seconds each. The intervals appear to be completely random and have no connection to whether the menu is currently being used or not. There seems to be no way to reproduce the problem, as the plugin worked perfectly up to this day. No changes have been commited to the configuration immediately prior to the problem showing up. The only way to stop the bug from occuring right now is to remove the menu plugin from the panel/forcibly stop the xfce4-menu-plugin task, after which it can be re-added normally (this will, however, make the bug reappear as well).

System Specs: Xfce 4.4.0 using a generic kernel version 2.6.20, plugin version unknown.