Pixilated and grainy frames of video

Hi all, This is my first post. I've just started using SP3 over the last month authoring home movies. I'm using FCPHD exporting to Quicktime, stand alone and then compressing that 19mb file through compressor. In compressor I use 90 min fast encode. Pal, Mpeg-2, one pass VBR, bottom first field dominance, GOP closed, 5mps average with 8mps maximum with best motion estimation. When I veiwed the burnt DVD I noticed a random pixilation of frames. Random in that it only starts about 15mins into the DVD and then comes and goes varying in intensity throughout the DVD. While I was single framing through the DVD I also noticed that every 3rd frame was grainy compared to the previous 2 frames. Any suggestions would be appreciated. Thanks in advance, Platty