Disappearing gauges

From time to time my gauges disappear, at least the graphics within the frame of the gauge disappear and the content goes black. Occasionally I get flashing white pixels, a bit like a TV not tuned correctly.It only happens when it's cloudy, all's fine a clear day, and furthermore, it only happens when pointing in certain directions! I've tried using simple and detailed clouds but that changes nothing.I've got a Radeon 9600 256Mb and have updated the drivers.Wondering if anyone else is having similar problems.