I can never re-create this problem.
For me it seems to be when i've installed a new version (often the beta versions you send me Mark) and for a day or two my machine can act strange and then it settles. Its nearly always the Stage Output going black.
I am never concered as I believe it is my old machine just being a grouch! I think its the graphics card. From the info above I think its the older machines.
@knowtheatre you may find the problem has stopped, or will do in a day or two. If not then my theory will clearly be wrong!