Sorry, might look like a silly question, but
what about your hardware?
What footage format are you using? On what type of hard disk?
Maybe thats Where it’s going wrong?
have you tried working with your rushes on a different hard disk?
If you’re using the activity monitor try monitoring CPU activity when your footage is playing.
Another option, we’re in the Voodoo phase, I had once a playback problem, it got just a little bit out of sync, started every clip in slow motion, and then continued in the right pace but out of sync.
Anyway, my solution was to erase my FCP personal settings, and restart and it worked for a while.
But I have to admit this is not a long term solution, the problem comes and goes. To solve it I need a faster hard disk.
Sorry I can’t do more.