I think Lynx3d here is close to the root of the problems. To me it also looks like it's a memory corruption on client side. Objects (rocks, bridges, walls etc) start to disappear when i've been in a game for some time and swapping through different regions - that mean when lots of info has been hold in memory. Restarting game (that means flushing memory) fixes problem, so it confirms my theory.
I have monitored gfx card memory usage, and its never anywhere near maxing out when it happens. Seen it happening around 300meg already, out of 1gig. And the bug goes away too, when i just simply enter some buildin and come right out.