Non-deterministic encoding

I'm using ffmpeg + libxvidcore to transcode a video stream to QCIF MPEG-4. It works fine, except the resulting MPEG-4 file is different every time. There is no visible difference, but some of the P-frame sizes sometimes vary.
The ffmpeg decoding/scaling is producing identical results each time, so it must be the XviD encoder  doing something non-deterministic.
Any idea what it could be? I can't find any references to clock time, process IDs, or keyboard input in the source code, and I've run out of ideas. Any suggestions?
Because the application is very sensitive to bitrate, reproducible encoding is vital, so I need to get to the bottom of this.

Regards,


mkwan




      ___________________________________________________________
Yahoo! Answers - Got a question? Someone out there knows the answer. Try it
now.
http://uk.answers.yahoo.com/