Very rarely, maybe 2-3 times a year, and I'm usually playing daily. For me, it seems to happen in the map editor of all things, but when I'm modifying or messing around with the maps that have come with the game, or that I've downloaded from this site. And usually the big 200x160 maps. But since I'm not a map editor expert, it may be something I'm doing. It's never repeatable, and I'm playing on an 8 year old Toshiba laptop, so it may be I just need a newer computer
Steve