I just wanted to provide a note on this issue.

I saw lots of text getting garbled up all the time on the Radeon R7 240. It was certain letters that just got all messed up all the time. When I minimized the window and maximized it again or entered the tty and back again, it was gone.

Anyways, by chance, I got a new card, a R9 270x, which I almost never see it. I think I've seen the screen corruption two times the last month, while on the last card, I saw it like 20 times a day.

Other than that, I experience here that the Xonotic game takes down the whole Xorg server, something that never happened on the last card, but I'll request help on that in another mail.