I've seen something like this in OSD-based SYSPREP based booting, many a year ago. It was down to the machines themselves, as I've used a single OSD script to push down to 12 identical(-ish) machines, and 80% of them imaged fine, everything automatically, while on a handful of them I had to just click "Next", as sysprep obviously DID pick up the data from the SYSPREP.INF but no idea why it wanted it confirmed.
Regrettably, sysprep's debugging options are virtually nonexistent as far as I know (to the best of my knowledge at present), and I had just put it down to "something" on those devices (since it was always consistently the same devices causing this misbehaviour when I was re-imaging the same 12 devices...
So, check whether there's consistency in the devices that cause you this - possible that there's something down to those specific machines that can be potentially improved (BIOS versions potentially, could be anything is the sad part).
- Paul Hoffmann
LANDesk EMEA Technical Lead