I'm trying to make a DVD from an DV .mov file but am having a problem. It seems that on the keyframes/I-frames, the image softens, far more than in all B & P frames. I found this by loading up the MPG file into MPEG Streamclip and navigating via I-frames. It is visible as a pulse in the image every half second when the clip is played back.

This is definitely not an artefact of the original footage, and happens whether I choose FFMPEG or MPEG2ENC to create the mpeg2 files.

Is there something I can do with the quantiser to minimise this or is it something to do with using DV footage? (though trying to encode with iDVD does not introduce this artefact).