Screen error when I try to use VGA MONITOR instead of TV

[NLQ_TRUC]

The magic mirror is find when I use it with TV monitor through hdmi cable.
But when I use vga monitor to make the real mirror and I use hdmi-vga cable. The screen turn on and I can see the desktop. When it start mm, I can see it works for a few seconds, after that the screen is black and the power supply led turn off and on continuosly. Please help me solve this.