I've been slowing clips to 80 and 90% for quite some time, then using AUPitch set at 400 and 200 respectively to correct the pitch. FCP is now crashing routinely (during rendering and/or in Compressor) at the 80% speed, 400 filter settings. I'm not aware of anything I might have changed as the filter is saved as a favorite. I'm using voice audio files recorded with Audacity and then exported as aiff files. Any hints you might have would be great. Right now my work around is to re-record the vocals at 80%. It also works (sometimes) to export as quicktime movies and importing them back to the timeline. If you have any hints as to why this might be happening, I'd be appreciative.
Don´t know if it helps but you could try deleting your renderfiles.
Did this problem come after some time? If so; you could try deleting cache files, the waveform cache files, (found in Final Cut Pro Documents)
end if that doesn´t help, your prefs might be next to delete. (http://www.digitalrebellion.com/pref_man.htm)