No external hard drives attached, except when I remember to back up my machine (I frequently, but not always, plug it in when it is on my desk, but it is just for Time Machine).
Also, unless I am running heavy CPU things (XCode, VirtualBox machines, etc), I usually don't get a lot of CPU time like that.
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.
http://cl.ly/1s3N2r2X1u2J2K010H2i
There's no process running wild from what I can see in the Activity Monitor, but it appears to happen a lot less frequently or not at all when I don't have my external hard drives attached.