External Monitor blacks out

I have a Macbook Pro with a BenQ monitor screen that blacks out for about 2 seconds every few minutes. (the Macbook screen doesn't black out.) I had a USB-C to display port cable connecting the two of them - it's has been that way for years with no problem until about a week ago. I have changed the cable to a different port (USB-C) on the computer, bought a new cable (USB-C to display port), and connected to the monitor using a HDMI cable instead. None of these cured the problem. Any suggestions as to what to do next? Do I need to replace the monitor?