Hi stefan keta
I have the same issue with my VX229H!!
The problem appears especially the time when you use programs like" Fix Dead Pixels " to solve or fix dead pixels or burn in problems with periods such as 16 ms for a long time in the setting which increase the local temperature in some parts of the monitor as well.
Sometimes(as you mentioned )the line(s) may be concealed by doing nothing!!. I think that I can not fix it, and nor any other person because it is a structural problem in VX229H model since I have heard about this problem from other customers.
As we know the monitor may run into serious trouble with unexpected physical shocks, over heating .etc. but in this model after a while this may occur by chance!
I took mine to the"Asus Repair Center " to fix the issue and they told me that you have to change the panel but they can not describe it in detail.
It however is not related to the dead pixels ...
Search the YouTube to find some creative solutions which are based on how to push or pull some critical areas around the screen and panel but they are not panacea.
Next time I will buy MX series monitor.My bad I lost my monitor...