I have a 2017 Mac with a Radeon pro 580 graphics card. I am running two outboard monitors, one of which is an old Apple one. That one intermittently goes off and on, and I can figure a correlation of why. Does anyone know a way to test to see if the problem is with my computer or with the monitor. I don't mind replacing the monitor, but hate to do so if the problem is in the graphics card. Thanks