Monitor problem after system left overnight.

I have an older pc with an old slot II Celeron processer. It all seems ok. During the day when it goes into screen saver a small mouse movement brings it right back OK. But when it is left over night and I give it a mouse movement it comes back after a few min. with the screen seeming to vibrate and the mouse no longer will move. I can't do anything with it so I have to power down and restart it. It has an Nvidia MX-200 video card. Has anyone else run into this? Does anyone have any suggestions? Thanks Bob