From FierySwordswoman's post
Common Issues and Their Solutions
:
FierySwordswoman wrote: Firstly, The Depth Buffer turns itself off during network activity to prevent exploitation of competitive multiplayer games. This causes depth-related effects to either flicker or not work at all, and *may* even apply to single-player games modes with weird internal server setups.
So, its intermittent / flickering property could be just explained like this.