Well, as we already know, FCPX renders everything to ProRes422 (or other flavour, set in the prefs) in the background. When there is a mistake in the rendering process, the green frames occur. I know that there are several "solutions". I can change the lenght of the clip, I can force FCPX to re-render the section, I can switch on and off the stabilizer (to force FCPX to re-render the clip), I can even go to the project folder in finder and delete the render files of the project. But: that's not a solution at all. It happens again and again, especially when there are speedramps, slomos or speedups in my edit. When you have a client sitting next to you, or you have really short time to finish your edit (I am running a mobile TV broadcast system)... it's really annoying.
I could also re-encode my compressed h264, mpeg4 clips before importing to FCPX, that would also fix this issue, but in that case I could work with FCP7 again.
The green frames never occur when there is only i-frame footage like ProRes in the timeline, so I figured that it's some issue with the GOP-structure of highly compressed footage like h264. Obviously the software is somtimes unable to calculate and display the picture, when it comes to a frame that is somewhere in the jungle between two video-keyframes.
@mochaL, I don't think it's a performance issue. I own 6 iMacs (iMac 27" i7 3.4 Ghz, 16GB RAM, 10.7.5 with AMD Radeon HD 6970M 1024 MB Graphics card). (I'm renting them to TV-stations for mobile editing) The problem occurs on all machines as well as on my Laptop (MacBook Pro 15" (2012) 2.6 Ghz Intel Core i7, Intel HD Graphics 4000 384 MB).
For the moment I think Apple has to fix this. Please keep posting your experiences with this issue.