There isn't any particular reason for running beta of OS X. I can try that, I ended up just removing those scenes and will have to reshoot.
The original file format is AVI and I'm trying to go to MOV I think. However if I can export to AVI maybe that will help I dunno....
Here are some suggestions:
(1) Upgrade both OS X and FCPX to the latest versions. If we spend time chasing a problem that could have been fixed by upgrading, it is all wasted.
(2) After upgrading, delete all your generated library files and re-render the project. This will not affect any edits. Procedure: Select Library in left-hand libraries pane, then File>Delete Generated Library Files, select all checkboxes and delete.
(3) If you were using optimized or proxy media select those clips in the Event Browser (or just select them all), right-click>Transcode Media and pick optimized and/or Proxy.
(4) Select the entire timeline and then Modify>Render All.
(5) If it renders OK in the timeline then export to H264 using the above-listed settings.
Re AVI, this is a container format and does not reveal much about what codec is used. In general FCPX is very good at using camera native format for editing, but since you are having a problem I'd suggest any AVI content be transcoded to optimized media. This is not generally necessary, but just a troubleshooting approach. It can be done after import within FCPX as mentioned above.
If you want to share the final product for web, computer or mobile devices, exporting using H264 using the above-mentioned settings is usually best. That will produce an H264 .mp4 file.