XP Taskbar Has The Jitters

For about 50% of the time that the computer is running, the task bar is normal. For the other 50% of the time, it stretches to almost three times its normal vertical dimension and "vibrates" or "oscillates" . I don't have the proper word for what it's doing, and maybe a screenshot might capture the phenomenon. In any event, through the blur that can further describe the jittery taskbar, I can sometimes manage to make use of its functionality, albeit with difficulty.

What kind of a problem is this?

 

keep taskbar on top of other windows
show quick launch
group similar taskbar buttons
show the clock