Clocked Video Output Settings

DVI is not usually used for such low resolutions. The DVI spec has a minimum pixel clock of 25.175 MHz. To achieve this clock rate with such a low resolution would require a high refresh rate or unusually large blanking intervals.

The HDMI spec has a similar minimum clock frequency. For resolutions like 720x480i60, the HDMI spec requires pixel repetition to get the clock rate high enough.

What are you trying to feed with your DVI output?

Perhaps you need to consider scaling up your video to something like 640x480p60.