Media Encoder / Premiere Encoding Glitch D700

Hoping someone might have some ideas...  Adobe are pretty clueless on this. Encoding Prores 1080p from an 8k R3D Premiere Pro timeline produces very noticeable random vertical glitch lines ever 12 frames or so. This happens using Open CL, Metal or Software only,  whether encoding straight from the Premiere Pro timeline or via Media Encoder. On my system it occurred in both Premiere 2018 (12.0.1) and Premiere 2017 - but only when Maximum Render Quality is selected.

System is a Mac Pro 12-core with 2X AMD D700s and 64gb RAM, OSX 10.12.6. Trouble shooting advised by Adobe extended to deleting cache, moving project file to system drive and exporting to system drive location... and then they were pretty much out of ideas. The call from the senior tech never came.

I've been working with Premiere since 2013 and never seen this before. D700s were recently replaced by Apple.

EDIT: I booted the same machine off a backup of the system drive made after upgrading to Premiere 12.0.0 but before the update to 12.0.1 (and before the D700 replacement). Rendering from the same project, with the same output settings (max render selected) - on the same machine - no glitch. So I went back to booting from the internal system drive, uninstalled all Adobe products, ran Adobe cleaner, reinstalled Premiere and Encoder 12.0.0 from direct download links - and the glitch is back. I'm stumped - seems its not hardware, has the update from 12.0.0 to 12.0.1 done something irreversible to my system that prevents it rendering max quality without glitching?