


I can produce similar artifacts.

I am using a Centris 610 with an Apple 16" monitor.  I got the 8/230/CD
configuration, so there is on-board ethernet and 1 Megabyte of video ram.
The effect only occurs in 256 color and 256 grey modes.  Any vertical scrolling operation, whether with a scrollbar or, say, using vi and inserting lines,
will produce a scattering of horizontal flickering lines on the screen.
They do not persist -- each is visible for perhaps one refresh, and then
that part of the screen is back as usual.

They seem to always start at or about the 64th pixel from the left,
and are maybe 512 pixels wide.  This is certainly not preventing me from
using my Macintosh ;-), but it does seem to be a sign of something
perhaps grounded incorrectly in the video circuitry...


I went ahead and called the Apple Customer Assistance Center, at:
  1-800-776-2333, or 1-800-767-2775 (more direct?)

The person who answered was polite :-).  I also took the chance to complain
about my mouse button sticking, and about the mac hanging on boot after a
(hard - killed MacsBug) crash which left a CD in the drive.  (I had to play
with a paperclip for about 5 minutes to get thing to eject, after which the
mac booted fine.)