Sounds like a bad flyback in the crt. Those things
can be intermittant. Also, I've seen a bad vga cable
do strange things .. so it could be in the vga connector
either on the video card, or on the crt. One way to
test that is to turn the brightness control on the crt
up high to see if you have a raster. That has no logic,
and if you do, then it is at the connector .. cable or
video card.

johns