Another thing (and a long shot) - check the files really are CBR, by playing them on some other player that shows instantaneous bitrate as it's playing (e.g. XMMS). I've found that uploading a VBR file that's missing the VBR header is misrecognised as CBR based on the rate of the first frame. While you're at it, you could try seeking etc. in the other player, and see if it works there.