My glass ball is broken.
We need more informations, much more.
What CPU / GPU, what rfactor version, which mod, which track, which settings and which plugins.
Best thing i would recommend is to use rfactor lite and try it with the included mod.
If that works install your mod, try it with ISI track.
If that works too, add your track, try it with that.
If that works too, try adding plugins, profile, etc. step by step.
I am sure you ll find the issue this way.
If not, ask here again and give us more information.