LiteOn 451S Speed Issue

I am using 451S and Pinnacle Studio 8 to create DVD video.
I found that it takes around half (0.5) hour to write DVD +R in 2X mode. Where as it takes more than hour and half (1.5) to write DVD +R in 4X mode. I am using Sony DVD +R.

Sounds like a DMA issue.

Is there anyway to check whether it is DMA issue?
Is there anything could be done to fix DMA issue?
Thanks

Try this:
http://forum.rpc1.org/viewtopic.php?t=27678

I tried that Link, but my Hard Drive (primary IDE) is already in Ultra DMA mode.

Is there any other reason?

Thanks