We use MPEG StreamClip 1.2 on a G5 Dual 2GHZ, Media 100 HD 10.1 and OS X 10.3.8. Maybe you need to use Apple Software Update to update your OS and try to use StreamClip again. I think you can download an older version of MPEG StreamClip that maybe runs on your environment or try to delete the MPEG StreamClip preferences.
The only thing that doesn’t work for me with StreamClip is exporting with Animation Codec. When I do this, if video levels are illegal, i get some “posterized frames”, but I use Media 100 Pal 720 or 844/X 8bit codecs.