Yes, I know, but sometimes you unintentionally switch it (that’s what happens to me sometimes).
But… you’re Video Playback settings aren’t stored in your project!
You can change it, and next time you open FCP it comes back with the settings you left FCP with.
Easy Setup will store and recall the settings of Video Playback.
The bug is gone in FCP 5. The black stripes are only in the Analog output. The firewire and SDI output are clean.