Well, around half the clips. Not necessarily 50%, not that exact.
Basically, it's like covering your eyes. Sound, but no sight, the video shows as a black screen with sound and then works fine again, then it goes black etc. So, some of the clips on the timeline, when rendered, turn out black for no obvious reason.
It's very frustrating, and I have looked everywhere for a solution and/or a cause.

I will be getting a new PC soon, with all the features (2GB RAM, 4.80 GHz etc.). So whether that will fix the problem is unknown to me. Obviously, it's a very rare occurence, so I will see if I can upload a video to show what's going on.