If you happen to get renders that are jittery or glitchy it may be that you are using a template that uses a rendered 3D animation and your timeline is not set to the proper frames per second. Since 3D template video elements were created and rendered in a separate 3D software the timeline needs to match the render setting of the original animation.
To find out if your template is a 3D template visit the website and check the tutorial or the features list.
If you need to use a different frame rate for your project. I would recommend you do your animation first in its own project and then export the movie and then import it to your project and have FCPX retime it for you.
Please let me know if you have any questions. Have a wonderful day!