Syslinux - Jul 2006 - isolinux issues - only sometimes booting

till last week everything was fine, our cds with isolinux and created 
with mklinux worked fine.

but suddenly our build system fails, the cds don't boot anymore, or
more precisely: they do boot in vmware and on several new machines
we have, but on lots of other, older machines they suddenly stopped
working. we didn't change our scripts, and syslinux and mkisofs is
still the same version (debian sarge).

any hints on what we can try?

the build command is:
mkisofs -o image.iso \
         -b isolinux.bin -c boot.cat \
         -no-emul-boot -boot-load-size 4 -boot-info-table \
         -r -J \
         -A "somename" \
         -publisher "Some Name, Somewhere " \
         isotmp

the isolinux.cfg is quite simple:
DEFAULT install

LABEL install
   KERNEL vmlinuz
   APPEND initrd=initrd.gz
   ALLOWOPTIONS 1

DISPLAY menu.txt
PROMPT 1

any idea?

Thanks, Andreas