Csgo is an esport title, traditionally these don't allow anything but a 16x9 format monitor - I assume you've got an old CRT 4:3.
Typically, the monitor can be setup to run 1920x1080p signal with either black bars or via the monitor function regarding aspect ratio.
Try setting your custom resolution to a 16:9 aspect ratio, and then use your monitor's aspect ratio function to do the stretching.
Try one thing at a time. I dont know what you mean by overclocking your monitor - just try the above mentioned scaling method and go from there.