buffer underflow, any fix?

Hi

What's the reason and is there any fix for the buffer underflow problem?

Ie. when converting video, I get a lot of such errors:

  [mpeg @ 0xb7fc9860]buffer underflow

I'm already using VBR, like the following, why I still get buffer underflow?

ffmpeg -i "VTS_06_1.VOB" -acodec ac3 -ab 224 -ar 48000 -ac 2 -vcodec mpeg2video 
-b 3600 -maxrate 4500 -bufsize 224 -sameq -ss 0:6:50 -vframes 3000 
"VTS_06_1.mpg"

thanks a lot.

tong