Have you tried a different codec for your proxies, Cineform has a few issues at times on a PC.
Smart rendering is supported for the GoPro CineForm codec on Mac OS X. However, on Windows a few limitations exist.
On Windows, because QuickTime is not available as a preview file format, GoPro CineForm cannot be used as a video preview codec for smart rendering. Smart rendering GoPro CineForm is possible on Windows only when the frames don't require rendering.