I have been having the exact same problems….
My issue is a little different as I took “SD Shots” out of an HD video file…. aka the HD feed is 1920 x 1080, the SD equivalent is 854 x 480…. I created 3 new “angles” from each of the original HD steams and was left with 6 streams to do a multiclip edit with. Here’s my specs on the clips and my computer:
Clip Specs: Frame Size: 854 x 480; Vid Rate: 29.97; Compressor: ProRes 422 (proxy); Date Rate: Varies between 1.5 MB/s and 2.0 MB/s; Aud Rate: 48.0 KHz; Aud Format: 16-bit Integer; Pixel Aspect: Square; Field Dominance: None; Angle: each clip is unique from 1-6
Computer Specs: 2 x 2.26 GHz Quad-Core Intel Xeon, 6 GB 1066 MHz DDR3, OS 10.6.2, FCP 7.0.1
Thanks for your help!