Several weeks ago, I described a problem with my Apple High-Resolution
monitor and promised to summarize replies received by e-mail. To
recapitulate: Occasionally, every two or three hours or so, my monitor
momentarily loses sync. The entire raster image bounces down about an
eighth of an inch or so, then rights itself. This usually happens right
after I close a large window, highlight a large area, or do something that
radically alters the image on the screen. Sometimes, though, it happens
with no provocation whatsoever.

I have received a bewildering range of replies. At one extreme was
a man, clearly quite knowledgable about monitors, who said that the
problem was that the 13" monitor's power supply simply couldn't always
keep up with sudden changes of the raster image; the rapid change from
a largely light screen to a largely dark one overtaxed the monitor's
power supply. There was no fix available, he said, because nothing was
really wrong. (I guess he meant that the flaw was so fundamental that
it had to be endured.)

This seemed quite persuasive and I would have been tempted to
adopt an attitude of stoicism, except for a reply from a 13" monitor
owner who discovered that this very problem is documented in the 
Apple Service Technical Procedures, Volume 3, under the heading
Jitter Correction. The section describes a fix involving the replacement
of a capacitor, the adjustment of a potentiometer, or, failing that,
the swap of a circuit board.