Rendering by parts : Blocks not rendered or abnormals

1- in the past, I have submitted a bug (in 2.44, if I remember well) about abnormal rendering of some image blocks (appearing randomly on some unpredictable frames and image blocks) when rendering by parts, in heavy and complex scenes.
It was supposed to be solved, but I have encountered the problem again in v2.47. :frowning:
In my last short animation " Snow fall in early winter" , which is downloadable at the bottom of the following page, I used a lot of billboard particles for the snow flakes.
A rendering by parts without the snow flakes gave a good result, but with the flakes (50000 particles), some blocks of the image are not rendered on several frames, or rendered with an abnormal brightness level.
I rendered with raytracing, in Blender v2.47.
The material of the billboard particles have 3 texture layers with alpha : One blend halo with alpha, set to Orco, and two cloud textures with alpha set to Orco and Object.
The problem is mainly visible on the upper right corner of the image on the movie.
2- I have also noticed that the number of parts displayed in the line at the top of the render window is wrong :
If I use 12 x and 12 y parts, the number of parts displayed during rendering is 120 instead of 144. For 11 x and 11y, it displays 110. For maximum X and Y values, it displays 130.
Do you have any explanation for this ?

Nobody has encountered this problen ?

For question 2, see reply in other thread here.

Thank you for the informations !