I'm running VDR-1.3.33, vdr-xine-0.7.6 and vdr-mp3-0.9.13 (the latter with no source changes) and cannot reproduce your behaviour, at least not with just two sample images for two sample mp3 files.
I'm just trying to replicate your setup now. But the 0.7.6 vdr-xine won't compile, so am trying to get a new xine CVS to patch and complile, but the xine-ui won't checkout..? I'm getting "cvs [checkout aborted]: end of file from server (consult above messages if any)" xine-lib checked out fine.
Which version of mjpegtools do you use?
1.6.3-rc2
I remember a bug in mpeg2enc before mjpegtools-1.6.3-rc1 which failed to encode a test stream. But this is most likely not the problem in your case. To get my version of mpeg2enc working with the convert script, I had to add "-S 420mpeg2" to ppmtoy4m.
Yeah, I found the same and had to change the 420mpeg2. It's not that though as the mpg creates fine, just doesn't view properly in VDR/xine
Maybe it has something to do with xine: how do run xine?
I use sessions in gnome to automatically run the attached script. Nothing clever...