Attached Link: Firefly render bucket lines?

I am getting very noticable tonal changes along vertical/horizontal lines using the Firefly render engine - perhaps at the borders of the render buckets.

An example is a raw w.i.p. render I posted last night (link attached) - notice especially the vertical line above her left eye, and the horizontal line between nose and mouth.

Has anyone else seen this, and if so do you know how to correct it in Poser? I hate having to correct render faults like this in postwork, but maybe it's just a shortcoming of Firefly.

Thanks for any thoughts / advice.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)