Another issue potentially causing this could be your opengl/mesagl driver, possibly in conjunction with your graphics card.
Have you updated drivers recently? The newest versions usually bugfix.
I\'ve not noted any such problems with my GFX card, it\'s a geforce 5200 with 128 megs of ram, (agp 8x)
Another potential is that due to your system, the clipping is set incorrectly, so you\'re only getting part of the images. However, setting the clipping plane differently can directly affect performance (i.e. slowing things down substantially). What you may be observing with the \'floating\' trees (which I observed on an earlier install with the dynamic clipping set up) was that sometimes objects that slow things down too much disappear. I.e. tree trunks, leaving the tops, or bits of the temple, or buildings in the city. Ideally, you\'d have minimal clipping and a dynamic fog, but that involves hardware issues that can be totally incompatible with older hardware. An issue of \'breadth of compatibility\' vs \'cutting edge\'. You can either make things widely compatible, or you can use the best technology around. The one increases your base of testers, the other limits the test base to whatever you use, similar to optimizing for a trident card, versus optimizing for the Geforce Quadro 4. The one is very, very slow and limited, but widely compatible (it runs in VGA 256 color mode, thus any modern VGA card is capable of running it) The Quadro, however, in PCI-E(16x) mode, is extremely capable, but only runs on high-end systems, thus only the richest gamers, and top-end systems, can handle it, restricting your test base to a very small proportion.
My .02 tria
Lyistra