field render issue on long sequence render

We have some odd render issues on a couple of clips within a 2 hour timeline sequence on Media composer 8.9.1.

When we render the entire sequence to avid DnxHD 120i these clips exhibit parts of the picture moved sideways on one field, this over a second or so.

However when you render just the affected section the error is not there, indeed if I render the timeline in 4 half hour chunks again the problem does not show, but once again output the entire 2 hours and the problem shows up. It's not in random places but always the same point, but as I say never when you render shorter chunks.

Any suggestions ?

Media Composer 8.9.1

Windows 10 Professional

Z420