Terrible Artifacts on Render

Hi,

I have done several renders over the last while, and have noticed that a few of them have some really strange artifacts - the weirdest one of all, (see screenshots to understand my next statement!) contained flashes from footage that was not even in the composition. Almost a "tile" of faces from another project.

I will add a few screenshots of the settings, and an example of the artifact. It does not happen all the time - (thats what is pickling my brain at the moment) and not at all in TGA renders.

Thanks guys.

Thanks guys

Pierre