Sounds like a power surge. Is someone else turning something on near you?

thank you Network, I will try that. :smileyvery-happy:

What sort of screen is it? Some CRT (as opposed to LCD) screens have auto-degaussing routines which run when the monitor is first switched on - one computer I work on has an LG monitor which goes through this operation every time it comes out of standby. The display 'shakes' for a second or two while the degaussing routine demagnetises the screen, ensuring a crisp display. Is this perhaps what you're seeing?