Basically I had the same error with the display turning on and off over time. All in anger, I deinstalled the MagicMirror from the Pi and began at zero again.
After updating and upgrading the Pi and installing the MagicMirror, a new try with the PIR-Module was iniciated. So I installed the PIR-Module and somehow noticed that my GPIO cables were wrong connected (ground connected to GPIO21 and vice-versa).
So after this embarassing moment of disbelieve I changed the connections and checked if the sensor is working with a simple python script:

from gpiozero import MotionSensor
import time

pir = MotionSensor(21)

while True: #repeat this forever
    if pir.motion_detected: # if motion is detected print the message
        print("Motion detected!")
    if not pir.motion_detected: # if no motion is detected print the message
        print("No motion detected!")
    time.sleep(1)

Note that my pir is using GPIO21.
With this script I tested the sensor and it worked like a charm, even though the sensor is placed 5 centimeters away from the raspberry pi 3 (with no case or even a bead installed).
In the end, I only had to do a cd MagicMirror again, and install the MagicMirror:
npm install. With this setup the MMM-PIR-module works magnificient.

@deepshades
Well ok…
step 1: get nodejs and npm --> sudo apt-get install nodejs then sudo apt-get install npm
step 2: update + upgrade the pi --> sudo apt-get update then sudo apt-get dist-upgrade
step 3: install MagicMirror --> as seen here MagicMirror Github Page
step 4: connect the PIR-Sensor to your Pi --> ground to ground (e.g. Pin 6), VCC to 5V (e.g. Pin 2), Out to GPIO XX (e.g. Pin 16)
step 5: check if PIR-Sensor works --> as seen at my script above (watch your connected GPIO pin, maybe change in script is required) --> some fiddling with sensitivity knobs here is needed
step 6: install MMM-PIR --> MMM-PIR Github Page
step 7: reboot and reinstall MagicMirror in MagicMirror directory --> cd MagicMirror and npm install

this has worked for me…

@johnnyboy

The steps I described started after a succesfully installation of the operating system on the pi, so with no MagicMirror installed --> step 3 is the first time MM is installed.
These are the steps which worked for me, somehow after the second install of the MagicMirror (step 7) the PIR worked --> this step could be obsolete for others, thats just what i did experience.