fcpx loses audio & video preview

hi!
i'm having a strange problem with my fcpx: sometimes it loses the preview of files in event library or on the timeline, especially when i have a big project (>3hours +effects).
It also occurs on the timeline (only audio preview), i don't see the whole or some part of the audio file.
Does anyone knows how to deal with it?

You have a Timeline that is over 3 hours long? For what? Not even feature films are edited that way. Cut your production up into it's scenes. Put each scene into a Compound Clip in the Event Browser.

what do you mean exactly?
could you please write me in steps how should i produce my movies more professionally?

Cut your production up into it's scenes. Features are edited scene by scene, not a full 2 hour Timeline. Only after the scenes are done, then you assemble them together for output. It'll go much faster and easier. A 15 minute timeline works way easier than a 3 hour timeline.

You can make new, empty Compound Clips in the Event Browser, name them Scene 1, Scene 2, etc. Create a new Smart Collection based on "Compound Clips" or "Text includes 'scene'".

What exactly are you working on that is so long?

thanks a lot, i'll try this way.

my friend had a seminar from 12 till 7pm, after removing coffee breaks and sequences we didn't need, the movie lasts 4 hours and 6 minutes, and now we want to record dvd's and sell it.

that's also an issue: how to recorded now i made a new project, copied 2 hours of movie and burning on dvd...we'll see what quality i'll get in the end.

Traditionally you'd break that up into sessions. Edit each small segment by itself. Then drop those Compound Clips into Projestd for assembly. I'd recommend 2 or better yet 3 or 4 DVDs. But editing all 4 hours in a single Timeline is not efficient at all.