I'm willing to bet that your 8x burn didn't really work at 8x... use a piece of burnng software that tells you what the real burning speed is (DVD Decrypter, CD-DVD Speed, or turning on displaying actual burning speed in Nero), and you will find that your "8x" burn really isn't 8x. It will start out at 4x, and when the time comes for it to do a 4x->6x shift, it will attempt the shift and fail, and after that failed 4x->6x upshift, the rest of the burn will be continued at 4x; the burner will never try the upshift again. If you don't believe me, pay attention to the burning speed throughout. Or time the burn to see how long it was.
I also see that you're using US05. You may have noticed that this would not work at all with US0A or later.