step 1. go into /Library/Quicktime and move ALL the .component files out of there. step 2. Go into ~/Library/Quicktime and move ALL the .component files out of there. step 3. (re)-install perian step 4. Try playing a movie that usually craps out.

One of the annoying things about Mac OSX is that a rogue codec can mess up ALL media playing activities on the computer- so when you encounter problems, I've found the most successful strategy is to figure out which codec is messing things up.