Ubuntu 15.10 sees 2560x1080 resolution only sometimes ;/

I have a huge problem with my ubuntu 15.04. I have the monitor lg 29UB65-p. When I connect monitor to the launched laptop (i5 1st gen, NVIDIA GT330M, drivers 340.96), the Ubuntu doesn’t see 2560x1080 resolution and I also cannot manually add it. The highest resolution is 1920x1080. But when I restart my computer, the Ubuntu recognizes the resolution 2560x1080 and this situation is very very strange and annoying.

I tried to use this manual: http://community.linuxmint.com/tutorial/view/877 but I cannot complete this step: xrandr --addmode because ubuntu still thinks that my monitor cannot use the resolution 2560x1080 (but after restart it can ;/)