I also have this problem. I have flashed mine with the fast version of the first firmware that came out for the BENQ. Not the multispeed version. I can hear the drive spin down quite alot during play, and it sound like it struggles to read the discs. I also get dirty disc messages quite alot. I use verbs burned at 2.4 - 4x with new pioneer burner. Ive tried burning my backups with imgburn and clonecd, same result.
There seems to be quite a few people expiriencing poor performance from the benq.
Im all out of clues.

flash a slower version. I don;t think the benq (or any other xbox dvd-drive) can read backups at 12x so i think it makes no difference at all to flash a slower version.
At least i notice no difference in loading backups with the different firmwares, i now run 5x and it works fine, it also makes MUCH less noise