If you read my posts for the last two months or so, you’ll notice I’ve had no end to the troubles given to me by my girlfriend’s computer. One day, it started having black flickering lines that occured when you typed, or sometimes moved the mouse. I ended up replacing all the hardware, and it still happened! It was better on the new hardware, so she continued to use it.
I even posted the question on Ask.Metafilter! No dice. A few days ago, however, someone online sent me an email via my contact form, and asked if I had solved it yet. I had not. He replied back today, saying he had solved the problem.
Are you ready? The problem was:
xscreensaver 4.23
After upgrading to the new xscreensaver, everything worked perfect. The black lines are gone.
More information is available at http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=336368.