If you looked at the basic math
HDTV 1920x1080 = 2,073,600 pixels of detail
SD DVD 720x480 = 345,600 pixels of detail
If you don't create your graphics with the conversion in mind then you will have lines that are lost completely or worse reduced to single pixels and on an interlaced SD CRT they will flicker like crazy!
However I suspect that the reason they are flickering on your HDTV is that the reduced picture is being scaled back up badly along with the issue I highlighted above.
You need to do one of these things:
Scale the original graphics down in the Mac - adding a bit of gaussian blur - to about 0.3px to 0.6px should but you might need as much as 2px (without seeing your graphics I cannot guess). This will help to blur the hard edges which will scale better (yet softer) and stop flicker.
Recreate your graphics with softer edges and/or 'fine' lines that don't go hard-edged below 6px to 8px in size.
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