I suspect there's something else going on. Don't see how your video card could be causing this problem. Do you have any 3rd party plugins installed? You might try removing them from the plug in folder. Have you tried deleting your fcp preferences?
Do you want to edit one problem subtitle into a new sequence in a new fcp project and send me the project? Zip the project file before attaching to an email.
But for the next time, the best way to build subtitles is to use the default fcp text editor. If you use this, there are ways (if I remember correctly) that you can export the subtitles and convert this file to a .stl file that dvdsp can use to create a dvd with optional subtitles. You can also use this export to generate a subtitle list with timecode ins and outs for translation to other languages and importing back into fcp or dvdsp.
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