Bringing subtitles into DVD studio pro from a FCP XML and subs are shifting. WHY??

I have done subtitles in Final cut pro on there own track and when i send out the XML i am using title exchange (subtitle conversion program) to convert the XML into an STL (a dvd subtitle format that Studio pro understands) . Once i have the STL i am importing it into a subtitle in DVD studio Pro, all of my subs are coming in, but some of it is shifting and it isn't a general one seconds shift, it is more of a random shift. Can anyone tell me why there is shifting going on. I have subtitled in DVD Studio on it's own but it is so much slower to do then the quick duplication style that Final Cut pro gives me for subtitles. Any ideas, i know FCP is not a subtitling program, but the sub quality is not too important to the program..Anyone??