Are you sure it's not all the edges being cut off? Could be that overscan settings change when the resolution and refresh rate change. If not, try not to change the settings too much. A single number increase/decrease will likely have a relatively large effect. I know that increasing the total horizontal pixels on my TV creates dodgy lines everywhere so I steer clear of that setting.
As an example, with my GTS 250 the closest I can get (with the 15 minutes of effort I was willing to put in) is 23.97630 Hz, resulting in a frame repeat every 50 minutes. For this, I created a custom resolution with the default settings for 1080p/23.976 (which gave me 23.978xx Hz) but increased the total vertical pixels by one. I just checked and this measured refresh rate was shown by both MadVR and EVR so I assume it's accurate.