From time to time, the Unity panel becomes unresponsive and consuming 100% CPU. Sometimes this goes away after couple of seconds (3-20), sometimes it seems to just hang forever. Haven't found a reliable way to reproduce it. Looking at the debug output, I get:
** (unity-panel-service:6557): WARNING **: on_indicator_menu_show() called with a NULL entry
(unity-panel-service:6557): Indicator-Appmenu-DEBUG: Switching to menus from XID 37752738
** (unity-panel-service:6557): WARNING **: on_indicator_menu_show() called with a NULL entry
(unity-panel-service:6557): Indicator-Appmenu-DEBUG: Unable to get MWM functions for: 37752738
(unity-panel-service:6557): GLib-GObject-WARNING **: /build/buildd/glib2.0-2.37.6/./gobject/gsignal.c:2593: instance '0x8d60b30' has no handler with id '1106942'
(unity-panel-service:6557): GLib-GObject-WARNING **: /build/buildd/glib2.0-2.37.6/./gobject/gsignal.c:2593: instance '0x8d60b30' has no handler with id '1106943'
(unity-panel-service:6557): GLib-GObject-WARNING **: /build/buildd/glib2.0-2.37.6/./gobject/gsignal.c:2593: instance '0x8dbbb30' has no handler with id '1185159'
(unity-panel-service:6557): GLib-GObject-WARNING **: /build/buildd/glib2.0-2.37.6/./gobject/gsignal.c:2593: instance '0x8dbbb30' has no handler with id '1185160'
** (unity-panel-service:6557): CRITICAL **: panel_service_show_entry_common: assertion 'INDICATOR_IS_OBJECT (object)' failed
** (unity-panel-service:6557): CRITICAL **: panel_service_show_entry_common: assertion 'INDICATOR_IS_OBJECT (object)' failed
(unity-panel-service:6557): Indicator-Appmenu-DEBUG: Switching to menus from XID 31457463
** (unity-panel-service:6557): WARNING **: on_indicator_menu_show() called with a NULL entry
(unity-panel-service:6557): Indicator-Appmenu-DEBUG: Unable to get MWM functions for: 31457463
(unity-panel-service:6557): Indicator-Appmenu-DEBUG: Active window is: NULL
(unity-panel-service:6557): Indicator-Appmenu-DEBUG: Switching to menus from XID 0
** (unity-panel-service:6557): WARNING **: on_indicator_menu_show() called with a NULL entry
(unity-panel-service:6557): Indicator-Appmenu-DEBUG: Switching to menus from XID 79691783
** (unity-panel-service:6557): WARNING **: on_indicator_menu_show() called with a NULL entry
(unity-panel-service:6557): Indicator-Appmenu-DEBUG: Unable to get MWM functions for: 79691783
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<<<< between those messages panel-service blocks or rather doesn't respond and uses 100% of CPU
=============================================
** (unity-panel-service:6557): CRITICAL **: panel_service_show_entry_common: assertion 'INDICATOR_IS_OBJECT (object)' failed
** (unity-panel-service:6557): CRITICAL **: panel_service_show_entry_common: assertion 'INDICATOR_IS_OBJECT (object)' failed
** (unity-panel-service:6557): CRITICAL **: panel_service_show_entry_common: assertion 'INDICATOR_IS_OBJECT (object)' failed
** (unity-panel-service:6557): CRITICAL **: panel_service_show_entry_common: assertion 'INDICATOR_IS_OBJECT (object)' failed
** (unity-panel-service:6557): CRITICAL **: panel_service_show_entry_common: assertion 'INDICATOR_IS_OBJECT (object)' failed
I suspect this is the same as bug #1199877.
This is possibly a bad indicator or one of the myriad libraries in the indicator stack has a problem. Could you please list the indicators you have installed so we can narrow down that possible cause?