I am intermittently getting this problem: when I start an application, the taskbar (i.e. the part of the panel where icons are placed for running applications) starts expanding and contracting at a very high rate.  It is then impossible to see exactly what is in the taskbar and to interact with it.  At the same time CPU usage by the xfce4-panel process goes up to 50% and higher, and remains there.

This can happen anything from once a week to several times a day.  It does not seem to be triggered by starting a particular application - it can be anything (e.g. Terminal, Icedove).  Once it was triggered when a Python script I was running itself went into an infinite loop.  It does seem to be related to a highish system load, but this is not at all consistent - it can happen when the load is low, and it does not always happen when the load is high.

I don't know whether this is relevant, but the way I start most applications is with keyboard shortcuts I have defined, e.g. Ctrl-Alt-T for Terminal.  When I start an application in this way, and it is an application that allows multiple instances of itself to be run (such as Terminal), I will intermittently get two instances started at the same time, and when this happens the panel also starts its strange behaviour.  The "double instances" issue is merely a minor annoyance, but I don't know if it will shed further light on the panel problem.

This said, the panel problem is not exclusively restricted to starting applications using Xfce keyboard shortcuts: it has also occurred when opening a reply e-mail message within Icedove using Ctrl-R.

Sometimes this behaviour can be stopped by closing the application that appeared to trigger it, sometimes not.  Sometimes closing other applications will help, sometimes not.  Sometimes it stops of its own accord after a few minutes, sometimes not.  

I have discovered that I can get out of this behaviour by killing the xfce4-panel process, then restarting it.  When it restarts, I get the following error messages:
$ xfce4-panel
(xfce4-panel:4800): xfce4-panel-WARNING **: Internal plugins need the "X-XFCE-Module-Path" entry to work properly.
(xfce4-panel:4800): xfce4-panel-WARNING **: Failed to create plugin "minicmd"
(xfce4-panel:4800): xfce4-panel-WARNING **: Internal plugins need the "X-XFCE-Module-Path" entry to work properly.
(xfce4-panel:4800): xfce4-panel-WARNING **: Failed to create plugin "quicklauncher"
(xfce4-panel:4800): xfce4-panel-WARNING **: Internal plugins need the "X-XFCE-Module-Path" entry to work properly.
(xfce4-panel:4800): xfce4-panel-WARNING **: Failed to create plugin "smartbookmark"
I don't know whether any of this helps?

I have also discovered that when the panel gets into this spin, it can be stopped by opening the panel properties and changing it from 'Normal height' to 'Full height'.  This causes the vertical panel to expand to
fit the height of the screen.  The juddering stops, and the CPU usage of
the panel process drops to what I presume is normal: 1.5% - 2%.  (I find 
it a little odd that the process is not more idle.)

In short, this is a serious usability issue with little rhyme or reason to it.  I would be happy to investigate further if given some pointers as to what to
look for.  I am running Debian lenny, and I have reported this to the Debian bugs system (reference 611326), but have had no response whatsoever from them.