Hey, it's me with another problem (I really should learn to fix my own problems, but I have you)

. Anyway, it's kind of a minor thing but it's annoying. Every time I leave the computer and come back later, the moniter is all screwed up. The picture changes size and position; sometimes bigger, sometimes smaller, but always different from what it was when I left. I know some of my friends' old monitors have had similar problems. Even after I centre the sceen again, the splash screen on boot up is always up and to the left. I thought of two things that could be causing this:
1. Maybe this is happening because the monitor is kind of old
2. Maybe somebody in my house is playing a joke on me
I ruled out 2 because nobody in my house knows how to change the screen.
Does anyone know why this is happening or how to fix it?
P.S. The monitor is a Hansol B17BL