That is what I was afraid of 
sudo nano /etc/apt/sources.list
remove all and add only this:
deb http://repo.pureos.net/pureosrepo pureos main
deb http://repo.pureos.net/purism stretch main
(save it, Ctrl+X, type y and hit Enter)
after that run this command:
sudo apt update && sudo apt upgrade
(reboot and see if flickering still exists)