Are there any other output clocks from the DM355 available for measuring that are derived from the same reference crystal as the clock used for the video encoder? If so are you able to check the frequency with a frequency counter (note, an oscilloscope is not accurate enough to do this). Any variation from the expected output of the measured clock will be the same percentage error as the encoder clock.
Variations in crystal frequency are usually caused by the load capacitors not being tuned to the PCB parasitic capacitances, causing the nominal crystal frequency to be incorrect (and potentially also reducing the margin for stable oscillations to be reduced [there is an oxymoron if there ever was one!!!])
BR,
Steve