Mac mini video goes black

I'm running Yosemite on a mid-2010 Mac mini, attached to a ViewSonic monitor. Frequently the screen goes black. I jiggle the mouse and press keys on the keyboard, and eventually the video comes back, but sometimes only for a few seconds. I don't know if my activity brings the video back, or if it's just random. Sometimes the screen goes black so rapidly that it's impossible to use the computer. Can anyone help?

Thanks-

Robin