I believe the preview is controlled by what used to be (and maybe still is ) called the "video overlay". I would guess your video card is either getting too hot or on the verge of a collapse.
Thanks guys for the suggestions - I really hope it's not the hardware :)
It only happened twice so far, but I had to close and re-open Vegas to get rid of it. Interestingly, the video in Trimmer still played back perfectly well, when this happened.