I had the exact same problem with one of my monitors as well. The best suggestion I can give you would be to put those commands into a script file. I named mine
fixres.bsh and put it in my home
~ directory. Using your code below, I added the interpreter line as the first line, then added your code.
xrandr --newmode "1680x1050_60.00" 146.25 1680 1784 1960 2240 1050 1053 1059 1089 -hsync +vsync
xrandr --addmode VGA1 1680x1050_60.00
After you create the file, make it executable:
sudo chmod +x fixres.bsh
then add it to your start up.
Press the SuperKey (Windows Key) on your keyboard to bring up the dash, then type in
startup applications to find the Startup Applications app.
Then once in the app, click the Add button
The add the following line to the command:
bash -c "sleep 5 && /home/username/fixres.bsh"
I like to give it a sleep of 5 seconds at startup to allow other applications to complete loading.
Click Save when completed and close the Startup Applications app.
Hope this helps!