I tested some media that was burned immediately after the last Verbatim which was labeled disc 14. Disc 15 was a Datawrite Grey FUJIFILM03 which exhibited the same behaviour. However the next Datawrite disc labeled 16 was fine. However I am unsure as to whether I slowed to 4x from then on or if that was the point at which I had the drive replaced. I tended to burn all the Datawrite Grey at 4x anyhow from then on out of caution.
On my 3540 though I burn them at 8x with excellent results. I believe these can easily be oversped to 12x