It would be nice to have the outputs for:
$ grep -i "mode" /var/log/Xorg.0.log
You shouldn't have to --addmode, it should already be there(if the card supports it, and the display supports it). Although I'm finding out that nvidia cards are far less informative relative to my ati card (that failed). At least when it comes to the proprietary driver.
For example on this ancient laptop with an intel video chipset:
Screen 0: minimum 320 x 200, current 1024 x 768, maximum 2048 x 2048
LVDS1 connected 1024x768+0+0 (normal left inverted right x axis y axis) 286mm x 214mm
800x600 60.3 56.2
VGA1 disconnected (normal left inverted right x axis y axis)
(current mode is marked by a * in the output of xrandr)
And this can change between modes:
$ xrandr --output LVDS1 --mode 800x600
And of course restore it by chosing the other mode
$ xrandr --output LVDS1 --mode 1024x768
Your parameters might be different depending on the output of xrandr. Like VGA-0 or HDMI-0 instead of LVDS1.