You not running anything that is taxing on the cpu, like a distributed computing application?
Try change IDE cable, diff IDE port, try without the lite-on connected, try as slave.
What chipset does your mainboard use? have you installed its' IDE drivers? if so, uninstall and try Microsoft default IDE.
Your burst rate is acceptable, i used to get 23MB/sec on my old mainboard and it could read to 16x fine. I did have a cpu issue though where it couldn't sustain a speed much higher than 12x.
Check that the screws you have used to secure the drive aren't long ones and not too deep in. If they're standard short screws, make sure they're secure and the drive isn't rattling.
good luck, i understand how annoying it is for you. after all the above i guess your best bet is to compare with another benq drive on the same ide plug...