Reducing speed often lowers burn quality with newer, higher speed media and burners, although there are exceptions. If the burner's firmware does not have good support of the media, slowing down may help, but it's probably not the issue here as it is Verbatim - well supported.
mmg, what is your model of burner? What burn speed are you using? Download Nero CD-DVD Speed and it will tell you the model of your burner, the firmware version (listed beside the burner model), and you can also get more info on your discs and test them as well. To make it easier to help you, list your burner, burner firmware, media code from CD-DVD Speed (such as MCC03, etc.), and burn speed you are using.