Yea. I'm having the same problem. I'm gonna try what you said, but mine get worse than that.
This map I made... I never checked it (i think it's around 30-50 r_speed average), but i bet when it's entirely, completly fixed it's gonna have r_speeds that will murder your computer.
So, the problem w/ it is that I can see down this canyon I made and see most of everything, except for the majority of entitites (and trust me, there's a HELL OF A LOT OF FUNC_BREAKABLES down there). And I have an APC, which I am very disappointed by, especially since it screwed the hell out of my texture alignment and i don't know squat about vehicles... anyway... when you get in the APC, it's fine. As you go further in, it disappears, then it might come back later if u look at it from another angle. Rocks will disappear in front of you etc...
What does all this evil mean?! Is there a balrog in the woodpile?!