Problems reading movie DVD's on my NEC-2500a

Hi all,
I bought a DVD and noticed it was freezing PowerDVD at a particular spot each time. So I ran Nero CD Speed surface scan, and sure enough, one red unreadble block appeared.

However, I ran the same scan through my old Creative DVD-ROM and no errors appeared.

Then I tried playing the DVD through my NEC-2500a again, but this time I lowered the DVD read speed one notch using DriveSpeed to 4x. (The default was the maximum 5x) The DVD played through fine with no freezes or glitches, and i’ve found that it plays through without problem at every speed below 5x.

I’m aware that the reading capabilities of the 2500a aren’t reported to be the best, but i’m surprised at the jump of being unreadble at 5x (not just damaged, but completely unreadble) to being perfectly readable at a slightly lower speed. I was wondering if anyone else has had a similar experience?