I open the screensaver and enter cycle of 2 minutes (time to wait until screensaver starts)

go about my business

suddenly nearly every window is deluged by fragments that flicker

had to use the computer reset button to stop the mess

All windows were useless to try to stop the screensaver or the daemon

Oh yes the cursor could NOT be noved!

Sounds like a graphics card issue - as the screensaver shouldn't start if your computer has activity.