[Solved] Creepy bug: the disappearing of track

For some reason, while working on track, adding objects and materials, the entire track just disappears in some places (or everywhere). I see only the car and the skybox. And few objects like roadsigns etc.
The same thing happens to the every track I work on, and I always had to revert to the previous version and do something again. But I cannot now. Because now I just don't have any previous version.
I have absolutely no clue what's going on, been re-exporting the objects with different settings for 4 days now.

Have somebody had that problem? What may cause this?

EDIT: Regrouping track objects and exporting with new names and poly count solves this problem.