Filmsg, export you avis as uncompressed. And where it says millions colors, change that to millions +. Also uncheck the recompressed box. Hope that helps.
I had this problem once. 16:9 then suddenly 4:3 - reason was it was doing the 4x3 when I applied transitions only as the pixel aspect ratio in the video settings was set wrongly/ It was set to 1 x 1 pixel and should have been !6:9 Widescreen. You must change this. Email back if this helps and you need more detailed info and I will take you through step by step...