Sweet! Thank you bepeg4d, that seems to have done the trick.
Now back to rendering........

Hi All,
OctaneRender v3.07 (yea adaptive sampling!) Cinema4D R17.055
Here are my render settings:
RenderSettings.JPG

So I've got a 240 frame animation that uses a key framed sweep (extrusion along curve with a circle as the profile) to extrude some geo along that curve. The sweep is key framed ...