Sorry for the necromancy but I have a good reason.
After some trial and error I have successfully found out the cause behind map tearing, and a very logical one at that. The name of the offender is of course filtering, as soon as you turn it off (completely), close down the game and reload the map, the tearing disappears.
Maybe some of you don't get annoyed by tearing, but I sure do, especially when playing beautiful maps like Dawn of Dominions. Fortunately I can now enjoy those maps to the fullest although without the filtering effect. So I'm extremely happy.
I now feel confident enough to report this as a graphics bug.
Rig used when testing:
Graphics: nVidia Geforce 6200
Driver: 173.14.12 (Linux)
OS: Fedora 9