[Cube] Need help with Cube gfx problem

[Dan MacMillan]

Hi everyone ...

I have had a G4 Cube for a number of years and it's worked flawlessly
until now, and I need some help.

A few weeks back the system began displaying the black "You must restart
your computer" screen. If I attempted to reboot immediately, it would
revert to that screen before completing the reboot, but if I let it sit
for a while, it would reboot successfully and work for a while before
displaying the screen again. This problem seemed to get progressively
worse, and the time I was able to actually use the system before that
screen popping up lessened to a few minutes.

I suspected this was a thermal issue of some sort, and when I opened the
Cube and discovered the fan on my nVidia GeForce 3 was seized, I thought I
had found my problem. I pulled it, and reinstalled the original Radeon,
and the machine booted normally. However, after using the machine for 15
minutes or so, the gfx flipped out, and this is what I saw:

http://www.geocities.com/dan_macmillan/gfxproblem.jpg

The small square near the middle is the cursor, and it moves with the mouse.

After using the computer for times that vary between 5 minutes and an hour
this screen always reappears. What's interesting is that only the gfx are
messed up; the Cube contines to run as normal in the background.

The room is always kept to 72 or 73 degrees.

Does anyone have any suggestions as to what might be causing this?

Thanks!
-Dan