I can’t imagine it’s a codec or media issue. When no external display is connected everything plays back perfectly fine. I transcode all my footage to Prores 422. Running OSX 10.5.8, latest version of FCS and QT 7.6.6
My scratch is on a external Buffalo harddisk connected through USB2.0, haven’t had any issues with that configuration so far.
http://www.youngindiefilms.com