Flashing frames on my timeline with 10.0.6

I have to admit to having a week or two of enormous frustration with the latest version of fcpx. Amongst moments of constant crashing when I have tryed to bring in a title made in Motion the latest issue concerns a compound clip I made from some 320x240 size video put on top of a piece of background video with a frame size of 1967x1080. The project is pal anamorphic 16:9. There is a screenshot attached. I would have thought this pretty simple and straight forward but 3/4 of the way through playing this clip there is rapid flashing between black and the clip. This has happened several times during this edit with other pieces of 320x240 compounded with the background video. It's easy enough to fix....I simply relay the top layer of video and it's gone.....but why is this happening in the first place? I'm running a 2011 Imac with 12gig ram, external firewire raid drive. I have to say I don't remember having this many difficulties before this upgrade.