Okay, so you're right - there's something going on here. Although my 800 is stable and i got a quick flicker in 400, just once at the start of recording. There was a fix to the original firmware that 'stopped' something that sounded like this. But clearly hasn't. Although it's not something i've seen in the footage i've shot - so i can't say it's always happening.
I've gone to the source of the files and i believe there's some scaling to the channels that is the cause, in which case there's hope to fix the data before debayer if it has caused a problem.
In the DNG's there is a value called black level, which is subtracted from the image. I see different minimum values for this across two DNGs and i can compensate for that in RAW Digger to see. Even doing that i believe there is a scale going on with the channels.
If it's something that can be sorted it would be best to sort this before debayer in the RAW DNG files.
I will have a word with them to add to any other reports sent.
What we should work out are the circumstances under which the camera does this...
EDIT: So i've passed on the files to sigma and see what they say. Useful for others that have similar files to do the same.