Try deleting your render files and re rendering, I often find it may be as simple as that.
If that does not work try using Digital Rebellions tools.
let us know how you go.
I'm surprised others don't have this same problem. It all started after the upgrade. I just can not function with FCPX anymore after that upgrade! I have no idea. I'll have to call or take it to the store. It's bizzar! It keeps on crashing.. I even removed my old events and projects from it and started fresh.. Still the same, even after I created a new event and imported some new footage and made a new project!
Have you tried running Disk Utilities on your system since upgrading? Worthwhile trying repair persmissions and repair disk.
Also try creating a new user and see if the same thing happens, this will show whether it is an app or system issue.