[same answer to same question as posted in the PowerMac forum]
This is probably completely obvious and maybe you've already tried it, but it should work best if you export to DV format (compression is fixed), choose DV-PAL or DV-NTSC according to your source material. Set audio to "no compression" and 48khz.
Source movies smaller than the native size (720x576 for PAL, 720x 480 for NTSC) will be scaled to fit.
Thing is, if you make a preview and that works, but you are still burning coasters, there is most likely something wrong with your DVD-R media... or possibly the hardware..?
hope this helps,
wulf