Mencoder and FFMPEG strangeness

I have a directory full of image files and I’m using Mencoder to convert them to an movie:

mencoder "mf://*.jpg" -mf fps=25 -o out.avi -ovc lavc -lavcopts vcodec=msmpeg4v2

This use to produce a good quality avi file that was reasonable in size. For some reason Mencoder no longer is kicking out good videos at the default bit rate. It 's not until I jack the bitrate up to around 8000 that the file stops being pixelated:

mencoder "mf://*.jpg" -mf fps=25 -o test.avi -ovc lavc -lavcopts vcodec=msmpeg4v2:vbitrate=8000

But this is producing huge files (~20MBs for 19 seconds).

FFMPEG is doing the same thing. My MPlayer install hasn’t changed lately, and the version of FFMPEG I’m using is from 9/20/2006. This is new behavior for this box. And it happens with any codec. Any thoughts?