PBG4 screen strobing and polarizing

I've been having a problem recently with my G4 PowerBook (OS X 10.3.9):
for no apparent reason, sometimes the screen will start strobing and
flickering and the desktop colors (Aqua Graphite) will pixellate and
throb. Sometimes the screen will go completely dark, except for a thin
gray horizontal band about 1" up from the bottom of the screen. This
happens most often on startup, but more and more lately it goes bonkers
when it's been up for any length of time.

I use Xupport, and when I run that, the problem stops immediately upon
restart, but eventually, it does it again.

None of this psychedelia seems to affect functions -- when I can SEE
dialog boxes, etc., I can still type, click, track, whatever.

Any help, suggestions, or voodoo greatly appreciated.