Credits not fluid after exporting project, help!? (CS3)

Hi!

I'm having a bit of a problem when trying to export my project in Premiere CS3. In my project I've created a title with rolling credits. If I play the video in Premiere the movie plays fluid and looks quite good. The problem, however, occurs when I export my project to a *.avi/*.m2v or *.mp4 file. When I do this the credits which looked perfectly fine before stops rolling fluidly from top to bottom instead they kind of jumps upwards in small steps. No big jumps but enough for a viewer to notice. Am I missing something when I export the moovie should I look to change the fps of the renderd video or is a codec I should try. I'm kind of new to this but I'm learning. All help is appreciated.

Thanks
/Skorpan