I get the "cannot allocate video buffer" occasionally, and I don't have an integrated card. It starts with buttons and icons disappearing, and that's when I shut it down and restart it.
I know that in discussion's of Dianthus' Mapfinder utility, she has it built in to restart conquests every X number of times, for exactly this reason, I believe. I know she(?) is running some 3.2ghz machine, so I think it's just a glitch with the game.