Can you tell us what speed you wrote that DVD with?
Otherwise I would say your scan is good - if written @12x on 8x media even excellent. Single digit PIE and PIF around 2 are very good imo. Small PIF evenly distributed, very well. I think you can ignore those PIF spikes - they are the reason the quality score is reduced to 97 and would be surely 99+ without them. Note that Nero CD/DVD Speed has a "green" zone up to 16 PIF IIRC, that means your highest PIF is not even half of what DVD Speed allows as "good".
About the speed issue, I am not sure, but the first drop (135-200MB) appears to be normal as everyone has it, even if scanning discs written on other drives. My BenQ also does some stops somewhere in the test - maybe the BenQ "collects" error scan information in the buffer which is then flushed to the PC? I dont know, but look at more scans and you will see such behaviour in most peopleÂ´s scans.
You might want to do the Transfer Rate (Read) Test in several DVD-Rom drives, the older and crappier the better I think. If they all show a nice smooth curve, congratulations to that writer/media.