Are you absolutely sure it not the media?
Changes in heat, etc after burning can result in disks that scanned as good soon after burning to look awful afterwards. Combine that with media of variable quality and you could easily end up with a situation like yours.
You really need to get a batch of different type of media (dont be tempted to buy the same type from the same supplier again bad batches of media can number in the 10,000s and you will probably get the same problems).
If the firmware is the same have you tried burning the same media on the other Benq and scanning it? This wont rule out bad media (some drives are simply picky) but if the scans are similar and especially if any PI/PIF peaks are mostly in the same areas it points to bad media.