[Ffmpeg-user] Running multiple instances of ffmpeg

Hi: I am using ffmpeg in a batch mode (Windows XP) to transcode avi to mp4 files. I noticed that only 30-40% of my CPU was being used during the processing so I decided to run multiple batch jobs. When ever I do this I see that one of the ffmpeg process still runs at 30-40% and the other runs at ~ 2-3%. Also, with multiple process running, one of the process usually has strange error. Is it possible there is some contention for a work file (or other resource). Is there a way to build ffmpeg or a command line switch to allow multiple ffmpeg process to run simultaneously? Thanks for any help. Mark