[Graphics] 2560x1080 resolution problem

[Guest]

Hey,
I have a LG 29UM59-U monitor which is 2560x1080 21:9 ratio. On windows it works flawlessly, on kubuntu and on linux mint - it doesn't. The problem is I can't even choose 2560x1080 resolution in 'display' settings. I've tried adding custom resolution with xrandr:

 erzis@erzis:~$ cvt 2560 1080
# 2560x1080 59.98 Hz (CVT) hsync: 67.17 kHz; pclk: 230.00 MHz
Modeline "2560x1080_60.00"  230.00  2560 2720 2992 3424  1080 1083 1093 1120 -hsync +vsync
erzis@erzis:~$ xrandr --newmode "2560x1080_60.00"  230.00  2560 2720 2992 3424  1080 1083 1093 1120 -hsync +vsync
erzis@erzis:~$ xrandr --addmode HDMI-2 2560x1080_60.00

My GPU: AMD Radeon HD 6870. I'm connected via HDMI-HDMI cable.

After entering these I'm able to pick 2560x1080 resolution and it resizes succesfully. The problem here is it is so blurry and buggy. What might be the cause?

[oshunluvr]

Modeline is slightly different from what gtf provides:

[FONT=monospace][COLOR=#000000]  # 2560x1080 @ 59.98 Hz (GTF) hsync: 67.06 kHz; pclk: 230.68 MHz[/COLOR]
  Modeline "2560x1080_59.98"  230.68  2560 2728 3000 3440  1080 1081 1084 1118  -HSync +Vsync
[/FONT]

What the output of xdpyinfo |grep 'dim\|reso'

[oshunluvr]

Also, LG says it supports 75hz.

[FONT=monospace][COLOR=#000000]cvt 2560 1080 75[/COLOR]
# 2560x1080 74.94 Hz (CVT) hsync: 84.68 kHz; pclk: 294.00 MHz
Modeline "2560x1080_75.00"  294.00  2560 2744 3016 3472  1080 1083 1093 1130 -hsync +vsync

[/FONT]

[FONT=monospace][COLOR=#000000]gtf 2560 1080 75[/COLOR]
[/FONT][FONT=monospace][COLOR=#000000]# 2560x1080 @ 75.00 Hz (GTF) hsync: 84.60 kHz; pclk: 295.08 MHz[/COLOR]
Modeline "2560x1080_75.00"  295.08  2560 2744 3024 3488  1080 1081 1084 1128  -HSync +Vsync

[/FONT]