Last week I upgraded my Radeon HD 6670 (Northern Islands) to a Radeon R7 250 (Southern Islands)

Since then I have encountered some strange problems in 2D.
Surprisingly, 3D works flawlessly (tested with Brütal Legend, Wasteland 2 and a couple of others)

Now, the first problem was that the system did not find out my monitor's native resolution. get-edid exits with some "Something special has happened". However, the monitor can't be the problem since with my old card detecting it wasn't any problem. (BTW: connected with DisplayPort)

Through xrandr, I got my monitor to work with its native resolution (2560x1440).
So I generated some modelines based on my monitor's specifications and manage to run X with these settings.

However, I still have many issues.

nedit: While typing, when the cursor moves the old one stays as a graphics artefact.
jedit: This editor was suddenly extremely slow. I managed to narrow down the reason to the properties line: view.antiAlias=none. It seems a disabled antialiasing feature makes the programm slow as hell.
console: The console still operates at a much lower resolution than possible.
gnome: When I start gnome, occasionally I only see a part of the screen on the top left corner. The cursor, however, can move to the whole screen.
switch to console: Seems to be much slower than before. It takes up to two seconds. Could be because of the necessary resolution change.

Besides that, X seems to freeze more often than usual. Sometimes the mouse can still be moved, sometimes not. The screen always becomes unresponsive.

Xorg.log does not show any errors.

Are any of these symptoms known problems of the open radeon driver for Northern Islands cards in 2D?

Thank you!

(PS: I am running Debian testing.)