Thanks for the suggestions.
I have tried several approaches to get a stable wifi connection. And that is actually the first problem before going to the magicmirror part, and was hoping to find someone who shared a similar experience.
I could never get a stable connection with the raspbian stretch. And following all the steps from the start (reformatting the SD card), and tinkering around with the wlan settings, has got me so far.
@Mykle1, were you able to solve the problem with wifi connection as it seems like the closest occurrence of the issue?