Rendering issue - grid of pixel holes in alpha channel

[mike_b]

The problem we are experiencing is that we are getting single pixel holes in the alpha channel appearing in a grid that corresponds with the corners of the render tiles. We tried changing the size of the tiles - and sure enough the holes became closer - i.e. following the edges of the smaller tiles. When rendering within Softimage the render is fine! However when rendering with Royal Render we are getting holes in the Alpha - not black holes - but with ranges around 0.99 instead of 1.
It is only when rendering using XSIbatch or XSI -render that it happens. I checked this with a cmd line render - using xsibatch and xsi - render - both also produce the holes!
We are getting this effect on both png and EXR files. All materials in the scene are arch mat shader, so there are no funny shaders.
We are using Softimage 2010 NLM win/linux licenses and the render farm is using Royal Render v6 RC2, although we dont think it is Royal Render causing the problem - it seems to be xsi batch that is the cause.
Can anyone give any ideas on this?
Regards,
Mike