I'm currently facing this problem in which my models suddenly disappearing in some locations of my level.I've made my level using WED. It's a city level where lot of models are added like roads,buildings and cars.

Sometimes when I reached the end of the map my car model disappears suddenly. How can I fix this? Hope you can help me.

This makes me woder...
Might your camera be a bit too close to your avatar and or your clip near setting is too high???

@TerraSame

I have it set on default. Even adjusting the value made no difference.
Here's a screen shot to give you an idea about the situation.

Do you have a hollow block as sky around your world?
Maybe, you just drive into a wall of it.
To avoid this you could check something like solid sky box in the block's properties or the surface's properties...

yes, I have a hollow block as my sky. Is it causing the problem? I haven't reached the skybox position yet so I'm not sure if it's the culprit.

i'm guessing you've set camera.genius to the car entity and the camera then moves into the bounding box of this model?

nope i didn't set camera.genius to it. I think it's a clipping problem. I cannot make big levels because of this problem. I really hope you guys could help me solve this problem.

The weird thing is, only the car model disappears but the road model stays as is.

I had also this problem on my driving simulator, but i don't remember why, it stopped happening

@painkiller

Please help me. How did you solved it?

I guess this is the point. The origin of the road now is closer to the camera, and so the road is rendered before the car.
Look into the build options in WED, and try different settings for the road, it should build as level... sort of...
I only use models as assets, and use terrain for the underground.