Try reflashing using the B7L9 trick (flash to L9, reboot 2x, power off, flash to V9, reboot 2x, power off). Also, try to reinstalling your burning software. If using NERO, use Nero's General Clean Tool to uninstall it and remove all remnants of the install.
Have you tried any other firmwares? Everyone's Benq 1620 seems to behave differently. Thus, your question is near impossible to get an exact answer for your drive .. b/c simply your drive an system are unique.
Generally, I get good results with B7T9, B7U9 (YUDEN000 T02 @ T03 strat), and B7V9 (WOPC off) for 8x burning. T9 is ok with 12x. V9 not so good. ... here again, this is just from my experience.
Just look at the media scan postings here ... no two drives / results are alike.