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.
The mess is most likely coming from your source footage. Try transcoding the footage and replacing it in the timeline.
Sorry for only replying now, it was a rough evening and night last night - Young Master Devereux is cutting his fourth tooth!
Thanks for the advice. Ill give it a go today. I did recently update our Samurai unit to allow recording in DNxHD, and I see that this is one of those clips - maybe there is an issue there. Thanks for pointing it out. Ill take a look and feedback later.