Sections of map disappearing

Hi, I've had a quick search of the forums but I have not found what I'm looking for.

I am in the process of creating a off-road map using gridmap as a canvas. I have created a large mountain using terrain editor, but as I'm driving up it large sections of the terrain disappear intermittently (screen dump attached for reference). It's usually the section I'm driving on, almost as if the camera goes into the ground, only happens when on the mountain. If I move the camera around it comes back but not for long. I've checked all the graphics settings and cant seem to find a fix.
Has anyone had this happen before and have a solution for me? Thanks in advance.

 

If anyone else is experiencing similar issues, once I restarted my computer it has gotten quite a bit better, may just be a performance issue.

 

Just save your work, and restart the game, and it should work fine, just how the terrain works, when it is big changes to it.

 

Thanks man!