ADV7393 NTSC at 30Hz (up from 29.97)

I use the ADV7393 in square pixel mode with EAV/SAV input and a 24.54MHz clock to generate an NTSC CVBS signal.

This mode is all detailed in the data sheet and works just fine.  I have no issues with this configuration and I think it verifies my hardware is functional, my I2C programming works and my digital data stream is correct.

Now the twist.

My implementation needs to lock this signal to a legacy 30Hz system.  I thought I could pull the pixel clock rate down to the 24.57MHz range and leave everything else the same to achieve this.  I was wrong.  At some point between 24.54 and 24.57 every monitor I have tried quits decoding color and we end up looking at a greyscale image.

The CVBS signal from the ADV7393 looks fine on the oscilloscope.  With careful measurement we can see the color burst and all the other timing scale by 0.1%.  The best I can tell, the ADV7393 is working fine at this rate.  It may be that monitors can detect this small frequency shift.  All I know for certain is the system does not work right now.  It would be great if an ADV7393 register adjustment might help.

I expect my problem is obvious to one of the video experts in this community.  I could sure use some advice, whether it relates to the ADV7393, CVBS, monitors, or clocking.