I recently bought a couple GoPro Hero3 Blacks and have been experimenting with different settings, frame sizes, frame rates, etc...
One odd phenomenon I've observed is that when using Compressor 4 to convert the native MP4 clips, certain clips comes out "stuttery" in a predictable pattern.
The problem occurs when converting any footage other than 1080p 30fps (this particular size and frame rate converts just fine), and it only occurs when converting to flavors of ProRes (converting to H264, for example, is also just fine).
In clips that are a problem (i.e. 2k @ 24fps, or 720p @ 120fps), the stutter follows the same pattern:
1 2 3 3 5 6 7 7 9 10 11 11
In other words, the every 4th frame is a duplicate of the frame before it, then the next frame is the frame after what the 4th frame should have been.
There doesn't seem to be anything wrong with the source clips, as I'm able to convert them to H264 and use them natively in FCP X without any issue.