Unfortunately the 1640 is a pretty horrible 16X DVD+R burner. There are always huge PIF spikes after the 3G mark, just like your scan, only often they are more severe.
On the other hand, the 1640 burns DVD-R at 16X beautifully, which is a little odd because benq is considered to be biased towards DVD+R (they ARE partnered with phillips, after all!).
At this point in time I would say you probably don't have a defective drive, but it's hard to tell.
edit - the 1640 burns 16X DVD+R media at 12X perfectly, you should try that.