Building Preview?

[Jessicablue84]

I was moving along without problems, but now that I am over a minute into my video, instead of the still images or videos, every few seconds a blue screen appears that says "Building Preview". It lasts for a few seconds and prevents me from seeing my transitions, still clips, etc. It happens all throughout the movie, but didn't start happening until I had a longer video. Can anyone tell me why this is happening? Are my video/picture files too large? Any way to fix this?

 

Thanks,

Jessica