Lost textures in 'top-down' view

I already asked about this here: http://www.fs2004.com/forums/index.php?showtopic=15447 but any further suggestions would be welcome.Using a 9800 Pro 128MB with dual monitors - normally have throttles, spot plane and top down view on secondary screen; virtual cockpit on main. After flying for a while (anything from 5 minutes to an hour), large blank squares start appearing on the top down view. They're coloured as they should be e.g. blue over the sea, but no texturing at all. All other views are fine, fps doesn't change.I'm running 1280x960 on each screen (same as desktop), with 2xAA set in Catalyst 4.5 drivers.