That is not a bug, Video memory will rise as you play, untill you have seen every plane type,texture in the game. It dosn't load everything at start up. Only the most used stuff.
When you alt tab out, I dump everything and then I do not preload it back after the alt tab. So as you start to see things the memory usage comes back up.
If you run out of VMem AH starts to dump items on a oldest used method to start swaping things.
There is a problem with some drivers when this occures, it is what is causing some people to see blank buffer for text.
I belive I have a new system in place that for patch 3 that will fix the out of video memory condition.
HiTech