Please can you try the following
Go to the nVidia Control Panel
- Go in the tab with a name that must look like "Frequency and monitor resolution"
- Check the box "Show HDTV classic format" (or something like that), it will now consider your monitor as an HDTV.
- Click "Apply" and close any window open from "Display properties"
- Go back in the nvidia control panel.
- Go in the "Frequency and monitor resolution" tab again.
- Click on "Add" to Add a new resolution.
- Enter 1200 x 900 + 32 bits + 60 Hz
- In the "Advanced Syncronisation tab" : Select Center image
- Put the same values as in the screenshot below (its in french I know yuo will need to copy the values exactly)
- Click Apply.