interlaced error when nodes are active?

I have completed a scene to be used in a TV commercial
the format is PAL 16:9

I find that rendering out as interlaced (turning on the field odd / even) gives a much smoother result.

When I set up the compositor and add a few nodes, I can no longer render as interlaced. The main console throws up an error saying it cannot locate the file

I am using blender 2.5 however this was also an issue with the previous version. is there something I’m doing wrong?

Can anyone shed some light on this issue

Thanks

From http://wiki.blender.org/index.php/Doc:Manual/Render/Video_output

Fields and Composite Nodes
Nodes are currently not field-aware. This is partly due to the fact that in fields, too much information is missing to do good neighborhood operations (blur, vector blur etc.). The solution is to render your animation at double frame rate without fields and do the interlacing of the footage afterwards.

thank you…I’ll follow that up