I followed suit and change my Preview file to DNxHD which is my mastering codec of choice these days.
Same drill. Immediate identification on restart. Maybe its just AVC Intra. I know when I work with Photo-JPEG files I bump into a few anomalies.
Tom Daigon
PrP / After Effects Editor
http://www.hdshotsandcuts.com
Mac Pro 3,1
8 core
10.7.3
Nvidia Quadro 4000
24 gigs ram
Maxx Digital / Areca 8tb. raid
Kona 3