cs 5.5 low quality exported avi file

i imported a avi file and recorded audio clips to voice over the avi.  the original avi file was a screen capture using Camtasia software.  when i edited the avi file i had to make cuts and copy and past and sometimes stretch the avi to accomodate the voice overs.  so after all is said and done the exported file looks like crap and you can’t read the words or make out the transistions.  How can i improve the quality of the avi on export?  please help

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