If the video is jerky only sometimes - this is often due to something else running on your computer at the same time ... antivirus scan, defrag, automatic updates etc. Try disconnecting from the internet and closing any non essential programs.
I usually do a restart before editing.
If it is jerky all the time it could be your hard drive is not operating fast enough -try a defrag. I export my finished project files and finished movies to an external hard drive when finished to keep my hard drive as free as possible and then do a defrag before impoting raw footage from my camera and starting a new project.

Hope that helps.