BENQ BL2400 Screen doesnt work with higher resolutions after installing Nvidia drivers on Ubuntu 14.04

I have a dual screen setup on Ubuntu 14.04 that I just installed. The graphics card is a GeForce 8800 GTS. When using t he default Nouveau driver all works fine displaying high resolution desktops on both screens. But when installing and changing to the Nvidia driver, my second screen (BENQ BL2400) appears as type unknown and can only display 800x600 as the highest resolution. I would like to fix this.

The Nvidia driver that I chose is version 331.113 from nvidia-331-updates (proprietary).
I also tried version 340.65 from nvidia-340 (open source), but that made me not only loose picture on my BENQ screen but also the whole desktop lag on the main screen. I am using 331.113 now, and the only problem with that driver is what is described so far.

Judging by http://www.nvidia.com/Download/index.aspx the right driver should be  340.65 (Geforce 8800 GTS on Linux 64 bit). It can be downloaded from here:
http://www.nvidia.com/content/DriverDownload-March2009/confirmation.php?url=/XFree86/Linux-x86_64/340.65/NVIDIA-Linux-x86_64-340.65.run&lang=us&type=geforcem

I did not try to install via this download.

Here is some lspci -v information about my graphics card:

01:00.0 VGA compatible controller: NVIDIA Corporation G80 [GeForce 8800 GTS] (re
v a2) (prog-if 00 [VGA controller])
        Subsystem: NVIDIA Corporation Device 0420
        Flags: bus master, fast devsel, latency 0, IRQ 43
        Memory at f2000000 (32-bit, non-prefetchable) [size=16M]
        Memory at e0000000 (64-bit, prefetchable) [size=256M]
        Memory at f0000000 (64-bit, non-prefetchable) [size=32M]
        I/O ports at b000 [size=128]
        [virtual] Expansion ROM at f3000000 [disabled] [size=128K]
        Capabilities: [60] Power Management version 2
        Capabilities: [68] MSI: Enable+ Count=1/1 Maskable- 64bit+
        Capabilities: [78] Express Endpoint, MSI 00
        Capabilities: [100] Virtual Channel
        Capabilities: [128] Power Budgeting <?>
        Kernel driver in use: nvidia

Some more info:

glxinfo | grep OpenGL | grep renderer
OpenGL renderer string: GeForce 8800 GTS/PCIe/SSE2

I used this guide to set things up:
http://www.binarytides.com/install-nvidia-drivers-ubuntu-14-04/

Some attempts to configure the available mode and set it:

#xrandr
Screen 0: minimum 8 x 8, current 2720 x 1080, maximum 8192 x 8192
DVI-I-0 disconnected (normal left inverted right x axis y axis)
DVI-I-1 disconnected (normal left inverted right x axis y axis)
TV-0 disconnected (normal left inverted right x axis y axis)
DVI-I-2 connected 800x600+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
   800x600        60.3*+
DVI-I-3 connected primary 1920x1080+800+0 (normal left inverted right x axis y axis) 531mm x 299mm
   1920x1080      60.0*+   50.0  
   1680x1050      60.0  
   1600x900       60.0  
   1440x900       59.9  
   1280x1024      75.0     60.0  
   1280x800       59.8  
   1280x720       60.0     50.0  
   1152x864       75.0  
   1024x768       75.0     70.1     60.0  
   800x600        75.0     72.2     60.3     56.2  
   720x576        50.0  
   720x480        59.9  
   640x480        75.0     72.8     59.9  
"

Seems like the Nvidia X Server sees my BENQ screen as DFP-0 (DVI-I-2), wich is DVI-I-2 above:

#xrandr --addmode DVI-I-2 1920x1080
X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  18 (RRAddOutputMode)
  Serial number of failed request:  33
  Current serial number in output stream:  34

Offcourse, this then fails:

# xrandr --output DVI-I-2 --mode 1920x1080
xrandr: cannot find mode 1920x1080

All help appreciated :)