I think this sadly is the reason for your problem, i.e. you overwrite some memory or something like that. Every now and then there is a "bug" or small problem where a terrain or an object gets clipped too early, but not the whole scene and esp. only after quite some time.
I would ask myself some questions such as when did the problem appear the first time (/ was noticed the first time), what did you implement or change in the recent time and open some old versions (that you hopefully have) and observe if the problem appears there, too.