Do you mean when you use the L key to speed up, it is jumpy? I have same problem even with a completely new fcp 5 project. I discovered as solution this week — DO NOT touch the mouse at all, while previewing at the high speed scrolling. It works much better that way.
Also change the settings under the RT settings on the timeline. Uncheck the Scrub High Quality setting, and also try reducing the playback setting from dynamic to high.
hope that helps