Problem is usually burn speed and/or quality of DVD media. Stick with good media from Japan...Taiyo Yuden, TDK, Fuji, and Maxell. Verbatim 8x should be okay, too.
Burn a disc at 2.4x or 4x and retest. Does it work in a stand alone DVD player? You may need to reload PowerDVD. Also make sure the primary and secondary IDE channels are running in DMA mode.