vertical lines appearing on screen

I have a G4 Cube, 500MHz, 60 GBs running system 9.2.2. I have an apple
studio display 17in monitor.

when I start my mac sometimes the desktop starts getting tiny black
vertical lines and dots appearing. They seem to collect in groups all
running horizontally parallel for about an inch. This happens all over
the screen. Also 4 vertical parallel lines appear over the cursor.
these line are about twice the height of the cursor and flicker
various bright colours. The extentions logos appear to stop loading
after the norton file saver extention logo and then computer crashes.

does anyone know why this happens?

I ran the apple hardware test 1.1 and it comes up with the error
message disp 13/2 (which is a Video Ram error) if there are USB or
Firewire device plugged into the computer. I get no error message if I
disconnected the USB or firewire devices.

Does a video Ram error suggest it's my graphics card?

Please Help if you can, I really appreciate it