I recently had an issue where desktop preview was stuttering on a rebuilt Mac Pro. I attributed it to the graphic card, but today discovered that this is a widespread issue with FCP 7.01.
I found the solution here on the LAFCPUG forums. It has to do with FCP 7.01 only (systems that haven't been upgraded are fine), and is a simple fix: a quicktime component needs to be replaced from the install discs. Five minutes and you're back up and running.
When working with the new RED quicktime codec, I've had an issue where rendering in Color would produce quicktime files that are full of gobbledeegook. After much searching, I found the solution