I have the same problems with the same setup. I have found the problem seems to go away if I unplug the Blackmagic Multibridge Pro.
It mostly crashes when viewing bins in thumbnail view when there is a lot of clips. The thumbails start failing to create correctly (appear with a negative effect) then not appear at all just prior to crashing.
I also get these messages in syslog at time of the crash.
Mar 1 15:45:13 edit3 [0x0-0x4b44b4].com.apple.FinalCutPro[68963]: *** error: can’t allocate region
Mar 1 15:45:13 edit3 [0x0-0x4b44b4].com.apple.FinalCutPro[68963]: *** set a breakpoint in malloc_error_break to debug
Mar 1 15:45:13 edit3 [0x0-0x4b44b4].com.apple.FinalCutPro[68963]: Final Cut Pro(68963,0xa031d720) malloc: *** mmap(size=268439552) failed (error code=12)
I have also adjusted the thumbnail cache to 100MB.