Monitor has black bars on each side after setting resolution using xrandr

I am using three monitors on my Kubuntu machine. Two are connected to the graphics card using HDMI and DVI. The third one is connected to the internal graphics unit using VGA. This setup worked just fine for month. Yesterday I moved my PC to a different room and plugged everything back in. The monitor connected to the mainboard only shows a resolution of 1024×768 pixels, while the other two working just fine in 1920×1080 connected to the graphics card.
I tried using xrandr to set the resolution to 1920×1080 for the third monitor. That kind off worked. The resolution is now 1920×1080, but it looks squished and I have two black bars on each side of the monitor.

The xrandr commands I was using are the following:

xrandr --newmode "1920x1080_60.00"  173.00  1920 2048 2248 2576  1080 1083 1088 1120 -hsync +vsync
xrandr --addmode DP-1-1 "1920x1080_60.00"

Auto adjusting the monitor does not help. Nothing changes. The monitor in question is a Samsung SyncMaster S24B150

Edit: I just wanted to add a screenshot of the problem, but thats not possible. The screenshot shows the whole screen with the correct resolution. Wich probably means the software settings are correct?