MacPro (2012) with 27" Acer S271HL monitor attached. If resolution is set at any other than 720p or the refresh rate is set to any other than 50 hertz the monitor goes black on an extremely regular basis, for only a few seconds at a time.

MacBook Pro (2012) attached to Acer S271HL monitor. If Resolution is set to anything other that 720p and the refresh rate is set to anyghing other than 50 hertz, the monitor goes black, for a few seconds, with great regularity. It then comes back on and then off again, for a few seconds, between 1 and 5 minutes later. Then off and then on and then.................... If I am on an app where I am typing something, it does not seem to affect the typing, since when it flashes back on the characters I typed while it was black are shown. Last time I used this Mac and screen combination it worked fine. Only difference is that then I was using a USB Apple keyboard and now I am using an Apple bluetooth magic keyboard. Went to local Genius bar, which could not figure this out. Any ideas, please. CC