Photon Unity Networking - Lagging / Jittering

Hey guys,
im a beginner in Unity and i need some help.

I created a game with the Standard Assets Car Controller and PUN. Everything's working fine, but the cars are lagging or jittering, something like that.
I already read some threads about this, but they couldn't fix my problem.

Does anyone knows a solution? :/
And do you need some code or something of my project to analyze that?

Thanks.
J927