Matt,
We put our 1080 30F footage into an SD timeline, let it scale itself, and we set our render settings under Video Processing to High Precision YUV, set Motion Filtering Quality to Best, and under Render Control set Quality to Best.
We never have the problems you describe, our SD DVDs are perfect.
At home I have a Kona LH, so I work in native formats, and let the card do the downconvert, again, no problems.
Have a wonderful day.