HI,
Are you working in SD and dealing with anamorphic (non square) media. If so, check the properties of the sequence clips on the layers below the text.
Is the anamorphic flag OFF. If so turn it on and try the export.
Please let me know if this solves your problem. We get the same occurring occasionally when working in PAL SD 16:9 and we are trying to replicate it for Apple.