Sometimes when I boot up, instead of my login screen, I get a screenful of vertical purple stripes. There is a simple workaround: I just switch to a Linux virtual console and then back again. Sometimes I have to do it more than once but eventually the normal login widget appears. It happens with both gdm and slim.
I am curious to know what causes this behaviour. I have an Intel 82810 (CGC) Chipset Graphics Controller (rev 03).