Are you sure your event file is only 666.66 MBs big, or 666 GBs? I had a very serious crash too because my event file had grown to a whopping 758 GBs and I was using compound clips a lot at the same time within the same event folder (this is for a 90 minute feature film). You can read about my FCPX adventure here:
http://forums.creativecow.net/readpost/335/37972
Eventually an Apple representative read of my tribulations on this CCow website, contacted me directly, and helped me get things back online (though I lost about a week's worth of work). The fix involved using one of the backup versions of the event. The key when making projects with FCPX is keeping the event files small. I made 2 mistakes, one was loading all the footage I shot for the film into one event (this sounded logical to me at the time I began doing it). Then I started doing everything in compound clips, which apparently bloats the hell out of everything... Maybe that was happening with your multiclips action.
I do use Event Manager as this helps speed up FCPX by not loading every single project and event when i open up FCPX, only the ones I need to work on.
I still find FCPX tough to work with. A lot of spinning beach balls and wait time even though I have an i7 iMac. Sometimes it takes 20-30 minutes from the time I hit command/quit until the program finally shuts down. FCPX also takes forever to load even using event manager.
I know there aren't a lot of solutions in what I wrote here, but I have felt your pain!
Jack Niedenthal
Microwave Films of the Marshall Islands
http://www.microwavefilms.org
iMac i7 using Lion 10.7.x, HPX 170 using only one 64GB card
I live on an island in the middle of the Pacific, YOU are my only resource.