I too now think it is the buildings to which my microfreezes are related to mostly. However I have the impression that it is not so much a graphic issue but rather how the buildings are handled in terms of memory management or something related.

Here my observations:

When I fly over land and in the distance I see these greyish areas where a small town is located no problem with stutters. When I approach the town and the first buildings pop up I get these microfreezes. When finally I have had all buildings in this particular town once on my screen I don't have a big issue with microfreezes for this town as long as I stay there. However, when I go on flying to another small town it is as if the buildings were never loaded to the memory once and again I get microfreezes until I had seen all buildings in this town once.

It is as if as soon as I leave one town the algorithm throws all building presentation out of the memory (I don't know which memory, RAM or video card RAM) and so I have to reload them again when over the next town and that this causes the microfreezes on my system. I cannot judge if this is the problem though. It just feels this way.

My system
Vista 64b
i7 920
6 GB RAM
Nvidia GTX 275 1 GB