Marshall, since a reboot fixed it, that implies a corruption, probably in either the video driver as it existed in memory, or in the constructions created between the game and the video memory locations. Only the latter is something you can solve. The former can POSSIBLY be solved by upgrading video drivers (Redneb97 -- do you have the latest drivers?)
In the case of the latter, it could be the drivers, or it could be the game. However, if the game, the problem would have to have been saved across iterations of the game. That would mean a memory leak of some kind, which you should be able to detect it (if it exists across all video platforms) on your testing systems. Theoretically, once the game exits, there should be absolutely no trace of the game left in live memory.
If there IS a memory leak, that might also explain some of the other issues people have reported (running out of memory, for instance).
At LAST! The greatest campaign board game of all time is finally available for the PC. Can my old heart stand the strain?