Intermittent problems with scene transition

My application is an animation of a planet using two different “cameras” (using gluLookAt) - one from up close, one from farther away. The cameras orbit the planet. Every 30 seconds I swap cameras. On some transitions from one camera to the other, I get a white “flash”, probably one or two frames at most. Not every time, though - maybe one in ten transitions. My clear color is black and I’m rendering a starfield background using a skybox. Pseudocode of my main loop follows:

glClear(color buffer, depth buffer)
animate both cameras
animate world
render world

The render routine uses the current camera. Most transitions are smooth, but with others I see the “flash”. AFAICT, it’s not predictable (like every 10th transition).

Anyone seen anything like this before? I can post code or any more info if it will help.