Quality scans on my 1620?

Hi all,
I’ve only just noticed that my quality scans when using Nero cd-dvd speed (3.55) seem to differ from most I see here. The read speed line seems to show regular drops, kind of like drive recalibration when burning (see posted scans below). Yesterday I had a good look at other peoples posts and only saw 2-3 similar, whereas every one I do is the same. It only happens when I scan at 8X not at 4X. When doing a normal transfer rate scan the graph is perfect (last pic is transfer rate of same disc in pic 2). Any ideas as to why my scans are like this? :confused: