Is the "Stop" point the same with the bad discs (i.e. at the same time mark), or does the stop point vary from user to user?
Have any users complained that the DVD is not playable at all? NOTE: there is a current issue in all versions of DVD Architect that prevent burned DVD's from playing in some dvd recorder/players (LG DVD Recorder/players is a brand that is often mentioned). Sony is aware of the issue, and has said they will eventually issue a patch for DVDA4 (current DVDA 4a does not include a fix).
Are you burning Single or Double layer discs, and are the problem discs Burned or Replicated?
It's interesting that it stops at the same point for those who have a problem. That could be related to how the disc was authored, or perhaps a bad batch of discs? Does the same problem exist (at the same spot) if you burn to a different brand of disc?
Are you burning your discs at MAX Speeds? How do you burners work -- if they start off burning at a slower speed, then kick in to a faster speed (which alot of burners do), I'm wondering if the point where they are changing speeds might be giving you a problem.
You mentioned in-house duplicators -- I was wondering if the "bad" discs are all coming from one burner in one of your duplicators. Can you track the bad discs to a single burner in one of your duplicators?