What you describe doesn't sound that strange to me. When burning at 8x, the speed isn't 8x speed all the way. It usually starts around 4x, then it gradually increases till it reaches 8x on the latter part of the burn. Also, during a burn it is not abnormal for the buffer to jump between 0% and 100%, because the burner has to recalibrate itself from time to time.
The qualityscan you posted isn't very reliable, since the scan was performed at max speed. Scanning at 5x gives much more accurate results.
Tip: don't use the printscreen button on your keyboard, but use the floppydisc icon in the top right corner of CD-DVD speed to save the screen. The imagefile created this way can then be attached to your post by clicking "manage attachments" when typing a post.