Had a similar problem a couple of months ago. Following the instructions on MagicMirror Wiki page, I enabled the Full KMS OpenGL driver instead of using the fake one. As my setup is vertically mounted, I used xrandr to rotate the display.
Finally, installed a cooling fan on Raspberry Pi. No problem since then.

@striiker I am having the exact same problem. Screen goes black every 6-8 hours with just the pointer showing. I have to reload/restart the Magic Mirror then. This started to happen after upgrade to version 2.4.0
I have upgraded to 2.5.0 but the issue remains the same.
I am still trying to figure out the solution; just wanted to share with you that you are not the only one facing this problem!